hm, well, you could keep the center borderlands for calculating movement but just forget about them having different terrains than the province they're in. so it would add to movement strategy but not add to terrain complexity.
as for your idea about an army defending from all invaders that move within its range, I like it. that way if you have two enemy armies invading, you could station your army closer to the stronger one and they'll get a greater preparation bonus if they engage them because they have used less move points to attack them.
but like you said, how to handle two or more armies attacking that area? I think that when you tell your army to defend the area you should be able to give them orders, to be applied when the turn commences, about which target armies to defend against and how to prioritize them:
---idea for detecting and engaging multiple enemies in one turn---
orders:
initiate recon
-(each army automatically initiates one free recon at the beginning of a turn, so this orders additional recons)
-(takes one movepoint, can be initiated multiple times until movepoints exhausted)
-(detects enemy armies in detection range)
-(factors in successful detection: terrain enemies occupy,
distance they are from your army,
their stealth statistics,
their preparation bonus,
your spotting statistics,
your preparation bonus.
amount of previous recons from this spot, this turn, by any friendly unit)
note: if that's too complicated, recon can simply stay automated, with detected armies displayed at the beginning of the turn, based on above factors.
select pursuit range
-(0 to highest move point number, not including used move points)
-(this is how far your army will leave its station to engage; 0 means they will only defend their station)
select targets from detected armies
-(untargeted enemies will only be engaged if they directly attack this army)
-(selecting no targets allows support mode or evasion mode)
select support objective
-(may select a single point on the map, within pursuit range)
-(if empty, will ambush any -detected- enemy that moves there. ambushes cannot be evaded)
-(if occupied by friendly or enemy, will join any battle that occurs there)
-(canceled if a target is selected)
prioritize targets
-(primary, secondary, tertiary, up to as many movepoints as your army has)
-(targets will be engaged sequentially based on priority)
-(movepoints are used on the way to each target; distance from army to primary target, then distance from primary to second target, etc. this effects preparation bonuses for each battle. if the next target on the list is beyond the remaining movepoints, your army will use the remainder to approach that target. all targets in range, which are given orders to move in the following turn, will attempt to evade when engaged)
-(unprioritized targets will be attacked after any prioritized targets, from nearest to farthest)
plot movement
-(select a destination; automatically picks shortest route, destination marked as 'nav alpha')
-(selecting multiple destinations plots them as waypoints; nav alpha, nav beta, etc.)
-(if your army is detected, targeted, and is within the pursuit range of the enemy that has targeted it, it will be engaged; if it is engaged while executing a movement, whether it is just starting the movement or is en route, your army will attempt to evade)
-(evasion will only succeed if at the point of engagement, your army has more preparation bonus than the enemy army)
-(if you win or evade the engagement, remaining movepoints will be used to continue along the plotted route)
this concept works whether each province counts as one movespace, or if each one has a center and multiple borderlands so it is 3 movespaces across. the one terrain type of a province would still effect the rate at which you use movepoints to cross it or move around in it.
---another terrain idea---
now as for handling terrain as One-Type-Per-Province;
each province is within one Terrain Region, like Amazonian Jungle(AJ), or American Great Plains(AGP), or Mongolian Steppes(MS).
each Terrain Region has 3 types of Regional Features that apply to all provinces in that Region; Topography, Climate, and Ecology. example:
Topography Feature:
AJ - Coastal River Basin, minor movement bonus, major trade bonus
AGP - Flatlands, good movement bonus, minor trade bonus, major defense penalty
AS - Flatlands, (same as above)
Climate Feature:
AJ - Tropical Coastal: minor movement penalty, good agricultural bonus, good exotic bonus
AGP - Humid Continental: major agricultural bonus
MS - Arid Continental: weather disaster risk modifier (weaker), slight agricultural penalty
Ecology Feature: (encompasses only natural ecology, not human effects on ecology)
AJ - Jungle: major movement penalty, disease disaster risk modifier (stronger), good defense bonus
AGP - Grasslands: wind power bonus
MS - Grasslands: (same as above)
these features do not account for cities or human effects like nuked wastes or terraforming; so here are some more types of features that do. they are not Regional; they could vary from province to province regardless of regionality.
Development Feature: (encompasses urbanization/civilization/wilderness)
AJ - 3rd World Rural: minor movement bonus, minor agricultural bonus
AGP - 1st World Rural: major movement bonus, major agricultural bonus
MS - 3rd World Nomadic: minor attack bonus (bonus from attacking from this area), minor defense penalty
Special Feature(s): (can be more than one per province, encompasses human effects and other sidenotes)
AJ - Jungle Clearing - bad exotic penalty, disease disaster risk modifier (weaker)
- Flood Risk - weather disaster risk modifier (stronger)
AGP - Water Table Pollution - minor population penalty, pollution disaster risk modifier (stronger)
- Water Table Depletion - minor maintenance penalty, drought disaster risk modifier (stronger)
- Major Freight Connections - good trade bonus
- Tornado Risk - weather disaster risk modifier (stronger)
MS - None, lol mongolia.
(end example.) a province would also have its basic statistics, like population, technology, various resource levels, and so on. there may be units, buildings, projects, and policies that can change existing features or add special ones.
this way of organizing terrain by Region simplifies things because if you're in a region and you see someone else in the same region you know they have a lot of the same bonuses and penalties.
Further simplicity: different Terrain Regions can have some of the same Regional Features, as implied by the region names (Mongolian Steppes sounds like it would have some things in common with Russian Steppes, American Great Plains, or African Savannah; same with Amazonian Jungle and Congo Jungle, or Rocky Mountains and Andes Mountains).
also, if this seems like too many types of features (topography, climate, ecology, development, and specials) it can be just topography and climate (regional), and development(local). the basic sum of bonuses and penalties should be displayed, and there can be little popups, advisors, and gamepedia entries that say 'this Terrain Region is useful for blah blah and useless for blah blah, such and such other provinces are in this Region, this Region is similar to these other Regions.' This way a new player can use Terrain Regions effectively even while they're still unfamiliar with them.
---disaster idea (terrain related)---
I dont know if the idea of disasters has been introduced; but since I mentioned disaster risk modifiers, I think stronger risks will just = population penalties because no one wants to live/work/invest in disaster prone areas. on top of that, if there were actual disaster events in the game (for say, certain provinces, or all provinces of a certain Region) then the risk modifiers could do the obvious by increasing or decreasing the risk that those events will occur in that Region.
I think the base modifier for these different risks should come from the planet's statistics, which would include things like, weather intensity (weather disasters), chemical toxicity (pollution disasters), abundance of water (drought disasters), geological activity (earthquakes, volcanoes, tsunamis, aka geo disasters). of course all these statistics can be relevant without actually implementing 'disaster events' in the game; you just assign the risks through features and those features effect population in a province.
Note that the Region Terrains are not balanced to make all of them equally useful. This will probably give them unique roles and uses in geopolitics (even the useless ones). Heavily terraformed planets may have more balanced region terrains since the terraformers may have designed them as such, for instance, a byzantium 2 type planet where everyone has a representational province might have terrraformed, balanced Region Terrains as such, but naturally occuring Regions and even Planets are not likely to all be 'balanced.' if you want to make starting play 'balanced' for beginners, just give them a combination of provinces that add up to a good balance.