To what extent can you. . . use terrain? Can you setup death boxes? Deploy troops into forests and let the enemy go right by while you spring the trap after they've left? Simply put, can my love of military strategy actually be USED or do I have to play by hp bars, hard counters, and command & conquer resources and "logistics"?
Terrain is a very huge factor. Your units line of sights are determined by obstacles in the way such as trees, houses, mountains, hills, whatever. It's a direct line of sight so even if they were standing at the crest of a hill they can't see directly under them for obvious reasons. Furthermore there's smoke that can obscure vision.
Then you get accuracy bonuses or debuffs for certain types of terrain such as swamp or forest or mountain. The maps are quite varied compared to old Wargame so there's big mountains and fairly large cities to fight through. Units can't really shoot through things and accuracy is determined by the movement of your unit, the movement of the enemy unit, your stabilizers, and the distance between them.
Helicopters can ascend or descend if you want them to hide behind stuff like buildings or mountains. You can hide your troops behind ranges to avoid artillery fire. Things like that.
There are HP bars but units take more damage depending on where they're hit (there's front, rear, side, and top armour). Damage is determined by the damage type of your attack combined with armour penetration if the unit has armour versus the armour of the location hit of the unit and its hp. Also units have morale which can go down dramatically and make them useless if they're panicked or even slightly stressed. There are also critical hits where units could take damage to their tracks or even get stuck in mud or maybe their optics were destroyed.
The only resources are the command points you get for holding terrain. You have to supply your units with fuel and ammo as well and that involves setting up some sort of supply line.
Is there anything similar to spectator mode, so I can watch the battle up close without worrying about commanding my units?
Nope. There's replays though.
In addition to online with other people, can you play a local game with just yourself and the COM(s)? I'd rather not have to play with people all the time. . .
There's singleplayer and skirmish.
Will discontinued and experimental equipment be available? For instance, the Comanche Assault Helicopter or those crazy gun prototypes that Red Jacket keeps trying to make for the army?
For obvious reasons not every single unit that has ever existed will be in the game but there are quite a lot. Me trying to list them would be insane but don't expect to see very obscure one off prototypes.
Do destroyed units stay on the battlefield? I.E. Will destroying the leading vehicle in an armored column while they're in the middle of a city force the remaining vehicles to either go around or blast their way through buildings?
No, it used to be like that in Wargame but it made it such a clusterfuck that Eugen had to remove it. The wrecks are still there just they don't impede units any more.
And as a personal preference note, are there any infantry units armed with P90's? Too bad they can't dual wield P90's Goldeney 1997 style!
P90 didn't exist yet in this timeframe.