I'm only a level 18 mage and I'm still in chapter one, but I can answer a few of your questions.
1) Spell cards can be bought from any vendor that carries mage type gear. The mages guild is an excellent location to buy spellcards, because it has four different vendors that can carry them, and not to mention all of the other mage gear that they sell, such as staves, carrier/summon/modifier cards, robes, hats, etc. But apart from the mages guild, most of the cities have at least one vendor that sells these items as well. Also, if you're wanting to buy the necromancer cards then you'll need to watch for unobtrusive citizens on street corners that sell them (they're considered contraband).
2) Same as #1
3) I've seen some people say that you should max out one or two schools as you level up, but I've had no problem taking critters down and I have points in all of my schools. I think I have at least 5 points into every school. I've only got a couple into my Summons though, because I won't usually summon more than 3 creatures at once, but again, I've seen some guys go gung-ho with the summons and max it out so that they can summon ten guys at once. Me, I'd rather have just a couple of guys that can draw off a few of the critters while I nuke hard and quick. For the record, fire is my favorite school, because I've found in chapter 1 that not very many critters are resistant to it, whereas it seems like quite a few critters are resistant to either frost, shock or death.
4) The mage's guild is South of Chezz (I forget the exact name of the city, but it's the big city that is right next to a second city in northern Hatmandor, and it's the more eastward of the two... it has the merchants guild in it as well) in Hatmandor. You likely ran right by the mage's guild during your progression in chapter 1 and didn't even realize it. You may have found a teleporter called Queen's Crossing (again, I forget the exact name), and it is right off of the teleporter.
Just a suggestion or two:
1) You're high enough in level that you'll be able to go for the master's staves... do it. They give you a high increase in damage for the given school. Get master staves in all of the schools and use the right one for the type of magic school that you're casting at the moment.
2) Use ricochet whenever you have multiple critters to take down (which is nearly all of the time). It is well worth it.
3) Use summons to keep the critters off of you. Casting with critters beating on you really takes the fun out of playing a mage.
4) Don't forget that you can defend with your staff during those times that a critter or two is beating on you. Also, staves in general don't do the best bludgeon damage, but they can still take a critter down if you're really in a tight corner. However, like I noted above, I'm only in chapter 1 still, so this may change depending upon the level of the critter.