Hey, ShadowXOR and welcome to the forums on here and to this awesome game
As for your questions:
1. It will never stop being repetitive. It can't. Walking back and refilling won't ever become diverse and interesting all of a sudden xD
As for the other part of the question: there's no general amount of unit losses per enemy encounter that one can call "normal" or "acceptable". Especially since the outcome of a battle can differ HUGELY depending on the parameters: player skill/experience, player units and their attributes, enemy units and their amount and composition, progress in the game concerning spells already available, leadership boni taken and so on.
To conlude this point: it is best to play with strategies limiting or better preventing losses completely. There are numerous strategies on this forum concerning "no loss campaign"s or "no loss challenge"s. You don't have to play a perfect 0 loss game but the strategies presented in these threads WILL make your life immensely easier. I highly recommend reading those.
2. As a beginner you should always pick the weakest fight you can. being a beginner is not the only reason for that, tho. The more important reason being: there is no respawn. The amount of fights in the entire campaign is finite. A weak monster ignored more and more will yield significantly less experience for a incrementally higher-leveled hero. Therefore: kill low stacks as long as you're still low yourself to get maximum experience from them.
On the other hand, it works the other way around, too. Stronger enemy stacks will yield significantly more experience. But you said it yourself: at a price; and again: it greatly depends on your setup and the enemy's to say if the losses outweigh the gain or not. A strong medium level setup with good spells can easily annihilate even "very strong" or "overpowering" (enemy heroes excluded, they're a different story) stacks while a not so good setup can even have problems with "slighty weaker" or "match".
Important: note that the "weak" "strong" descriptions of the enemy stack is ENTIRELY dependant on the amount of leadership YOU have. If you have 10,000 ldr and an enemy stack is marked "match", go to the castle and put half of your squad in the garrison; come back and you will find the same stack labeled "very strong" or "overpowering". It's NOT taken into account if the enemy units are good counter-strategic units to your setup and well put together or just cannon fodder. A strong female troop with well boosted Lake Fairies and Sprites (and respective support and ranged damage units) will completely annihilate an "overpowering" troop mainly consisting of units that are weak against fire, magic damage or have a poor hitpoints per leadership ratio like Ents or Skeleton Swordsmen. Whereas, when the same setup is facing magic immune opponents and strong ranged units behind a line of melee blockers the odds are very much different.
3. By no means do you have to stick to a race. That is not intended and you can use qhatever YOU see fit for your playstyle.
When it comes to army composition you also have free hand in that. You will, however, be limited when it comes to employing undeads and humans as the latter tend to piss their pants and be demoralized (HUGE malus in stats)by the presence of the former. You will see a few more of these animosities ^^
But in general, it nowhere says that the only way to go is to stick to one race. Moreso: [race here]-only armies are often inferior to mixed setups. When it comes to certain roles like range damage, tanking or melee striker you will find that some races just don't have any good of that kind.
Take demons for example: you won't find any decent ranged units in that race (catapults only make sense in very specific setups and with respective items and for the most setups they just suck) but you WILL find one of the greatest tanking units in the game (note: depending on your setup). And so on.
4. well, there are quite many garrisons. Soon you will have more garrisons on the map than you can fill troops in. However, they're far apart at some point and it becomes more annoying to swap. One solution is speccing into "Reserve" in the Mind skill tree. That gives you 2 slots to carry your most used substitute (or refill, whatever you like) units.
I for example like putting one sort of dragon and an additional melee unit in my reserves to swap them with a support or ranged unit if I am facing an enemy army that makes quick work out of unprotected units (arch demons instantly teleporting in front of them for instance). On the other hand, as soon as you have found a playstyle and your favorite troops you will rarely need more than 7 different units to contest the majority of the game, especially on normal difficulty.
5. Like you did. You save your game, try and then you should already see if it's manageable or not. Then you load and come back to that when you feel strong enough.
6. there is no such thing as missing something important or areas not being accessible aftert doing something, EXCEPT one. But you will be notified numerous times that you will not be able to acces this particular area (it's only one and it's independant from the "normal" world). And a second area but it only has one vendor. If you don't want spoilers I won't tell you which one that is, but don't complain when you can't go back xD
However, things like that can occur on a smaller scale (e.g. in single quests, by picking one option you make it impossible to pick the other one). But that doesn't change anything on the big scale. No worries, quest away !
Okay, I have nothing to do tonight so I wrote a lot when usually I just refer to the many threads that deal with these issues but I was in the mood ^^
If you have problems understanding anything or have further questions that ensue, ask away.