It is a rating to tell you how powerful the unit is in general. It also has to do with the bar for how many units it takes up on your active army.
Aka you can only have like 100/100(random number not actual) at a time on the field. Then when it gets down a bit you send reinforcements to refill it.
Not unless mentioned, which currently means no, there aren't any (there are units that have a magic resistance in general but not against certain types)
I assume there aren't such immunities, so you aren't screwed all of a sudden if you put points into fire magic and then face a Pyre enemy (like in your example), but like I said that's just an assumption.
I am not sure what you mean with your second question. There are different ultimate spells that do different things (you can read what they do in the lexicon) and even if there are similar spells in different magic schools, then they would still at least scale with different skills.
Also if you have more questions you can join the Discord server, there are usually more people there to answer questions.