1. How good am I going to have to get to bother playing multiplayer?
You don't need to be very good at all. I personally consider single player pointless; multiplayer is the core of the game. I'd say, at minimum, know how the UI works, and have a vague notion of how at least 1 nation plays earlygame(expanding against independents; rushing other players; troops dominant) to midgame(fighting other players; magic becomes important), and that's probably enough. You can look for newbie games and only play against less experienced players, if you like.
2. I know you can recruit local troops from conquered provinces. So to what extent is my choice of race a "starting package" versus a class that will define me for the whole game?
Nation choice is very much a "defining class" sort of thing, though most nations support more than one strategy. Generally, independent troops suck even more than your national troops, and won't be worth recruiting, but there are exceptions. For example, a nation with no archers may find it beneficial to recruit independent archers. It can often be worthwhile to recruit independent mages that have access to magical paths that your national ones don't.
Also, keep in mind that to recruit (most) units in any kind of quantity from a province, that province will need to have a fort, and any province with a fort will be able to recruit your national units.
Even in the "lategame", which is maybe 80 to 100 turns into a standard multiplayer match, where you've acquired enough magical diversity and research that you have many many tactical options on the table, nation choice is still very influential on your playstyle.
3. You guys keep talking about "chaff". What is the strongest troop you would categorize as chaff?
Hmmm, chaff is really more of a troop role than a troop descriptor, in my mind. Like, most human nations will start out using their infantry as the primary way to kill enemies, but eventually research into magic will cause them to become obsolete in this role, and they'll be relegated to soaking up fire and guarding the back row. Conversely, even the lowly makarta can become quite deadly with a few well-picked buffs.
Am I going to spend most of the game fighting with summoned monsters or are my starting elites good enough?
Any tips for what to research at first?
I've put these questions together because the answer is the same: it depends entirely on your nation choice and strategy. If you have a particular nation that you like to play, we could discuss these questions in more detail.
How quickly should I gobble up the independents,
As fast as possible. As a rule of thumb, average at least 1 province/month in the first year.
how concerned should I be with defense at this stage of the game?
At every stage of the game, I usually put only 1 point of province defense everywhere. In most cases, spending gold on troops is more worthwhile. However, as you are playing singleplayer, this advice is not necessarily applicable. It's very easy for a player to kill 20 or 30 gold worth of pd, but the AI probably has more trouble (though I have not seriously played dominions 4 singleplayer ever).
Anyway, I'm just one guy with my own views, so I hope other players will chime in if they feel the need.