I've just started playing this game, and am quite hooked. But, I've got some things I seem to have noticed that I'd like fact-checking on, and some full-on questions.
I've only played tiny shards, on my 3rd, so this may have influenced some things. Highest level Hero has been 11.

Observations

1. The computer seems to have access to far more resources than you, but uses them in stupider ways, at least on beginner. The computer player (as a "Local lord", I'm that early) seems to be able to insta-res hero's, and to put guards on nearly every province. I'm assuming that I'm not playing *that* badly...

2. Losing troops is devastating

3. Comments on classes, correct me if I'm wrong
Warrior-too squishy at low levels, needs to be nurtured, and is vulnerable to missile fire for a long time in terms of levels.
Scout-Nice class, but can't really use it's non-combat abilities otherwise it never really makes a difference. Decent spell slots.
Commander-good once you have bowmen so he doesn't have to fight, can troopspam hard enough to overcome combat hero's (especially if you get some good mercs)
Wizard-haven't played since tutorial, looks like he's only useful once you actually have access to 2nd level and higher spells.

4. Mercenaries. Eador seems to be me to be one of the few games where mercenaries are actually worth it.

Questions
5. (keeping numbers so people can reply) What influences availability of mercs/types of mercs?

6. How bad is going back in time for you? (assuming there is a reason to go back in time)

7. (remembered an extra one already) What influences base disposition of a region to you? I know events can do things, but one of my games I had an Elven region that absolutely hated me. Put down half a dozen rebellions, failed to put down a few and had to reconquer as well. Was there any way to trick them into liking me, or is it purely access to buildings? (got an amazing blueprint for a centaur region that was similar, some statue of wonder or something. +5 to attitude and a couple of other things)

Thats all for now, will post and edit OP as I come up with stuff.

1. Insta res available if you win battle, but hero is dead. You can do the same. In case player loses hero, he can bury him and hire new. You can do the same.
If you kill hero, he will have same penalty on ressurection, based on distance from demesne.
2. Losing troops is convenient.
Especially brigands and thiefs - they MUST die to not steal trophies.
3.
warrior - build forge, take chainmail. Against missile fire - take shield, stand in forest.
scout - pathfinding OP, sabotages - innate rank 4 spells
Commander - um, what? bowmen? beginner only. On expert - barbarian horde + webs and astral energies. Or swordsmen+healers into tactic+crossbowmen and horse archers with elven archers later
wizard - can do a lot of things with just Fear spell.
5. Level of Inn. Protip - hire merc before upgrading Inn - you will get instant reset.
6. In singleplayer - nothing bad. In campaign - you lose some highscore points. You can trade this for astral energy but usability isn't very deep.
7. Your karma level. Good ones don't like evil master. Also - guard type. For example, people love forest spirits and conquering neutral province will give -3 to unrest. If these were elves, and you are evil -4.

Hi, and welcome. It is indeed a deeply engrossing, but hard game. I can't answer all your questions with exact certainty, but I'll do what I can.

1) I do not believe the computer has access to more resources (at least up to expert, which is the "equal" level), however he may get a discount on guards and/or their maintenance, because you are right he has an amazing ability to have guards in all provinces, despite the cost. Many of those, though will be thieves and cutthroats, which are very cheap (but have bad effects - especially on karma, but AI don't care about that).

However I have seen him unable to put guards in a newly taken province on occasion for a couple of rounds, and he certainly can't insta-raise dead heroes. I've killed those enough times near his castle, to know that it takes the requisite 2-4 rounds depending on distance, before they are back in business. Again he may get a discount on resurrecting them, but it's not free either, as I have seen AI's stalled and stopped completely by the death of their heroes and an unsustainable economy early on, letting them just sit there with a few empty lands waiting for me to get to their castle across the map no matter how long it takes. They are clearly in negative income, and unable to get out of the spiral. It's rare, but it happens.

2) It's not really. It's almost impossible not to lose troops, and it's not such a big deal either. What is devastating is losing troops too much. If you lose all your troops all the time, it'll cost you too much to replenish them, as well as them getting no levels. Levels for troops are far less important than levels for the hero, but they do matter. Incidentally this is the reason a Commander is a hard hero to start with - he's strong enough, but the COST of all the troops he need to take advantage of his bonuses is astronomical. You don't have to avoid losing any troops, but in tactical combat prioritize already leveled troops for survival, and allow fodder units to take the risks that may win the battle, but cost the troop.

3. Warrior is not really squishy at any time, but until you find a bit of equipment and 2-3 levels there is an inherent risk, in that he needs to get into melee to help, and that IS dangerous, especially until you learn the little details of tactical combat.

Scout is a nice class. Not sure what you mean with "can't really use it's non-combat abilities", but patthfinding is an amazing ability - in or out of combat, as is (I've now found) exploration.

Commander is a very strong class, but as said above the cost is prohibitive - both buying the units and the upkeep. I know some people swear by this hero for their first, hero, but I can never get it to work with the economy....

WIzard is the strongest class of all late-game (both late-campaign, and late in each larger shard). While he is severely handicapped early on by the lack of higher level spell schools, you do find a lot of scrolls in the course of a shard, and he can put those to good use. Still he's not an obvious choice for first hero - particularly not early on in the campaign.

4) Yes, but beware the cost....

5) What level of "merc" building (Inn, Mercenary Guild, Traders Guild) you have unlocked and built influences the types of mercs potentially available, but which you actually get at any one time is random..

6) Very bad in terms of score, but there is NO other effect. In short, use it liberally as you learn the game, then cut back once you get better and actually care about the score, rather than just winning.

7) Karma, most importantly. If you have bad karma, elves among others will HATE you with a passion. If you have good karma, some evil races will also dislike you (but not as much I think). Some will hate you for invading them, rather than ally with them. Dwarves always seem to be furious with me, if I invade instead of making an alliance with them, no matter what, and Lizardmen, I think, also get a bit snippy at being defeated. But you can't do much about it until you unlock guards and buildings that improve mood. You can use "Extravaganza" ritual to improve mood a little in one province by casting it each time it runs out, but other than that - not much to do no.