I'm also working on an A. Warlock build. For whatever it's worth, so far I've played her ~10 times and seen her played ~5 times additionally.
Addressing your questions:
1). To me this is the most difficult question, I'm still figuring this out as well. Right now I generally have 1 copy of most high cost situational curses (deathlink, enfeeble, arcane corruption, etc), and multiples of low cost curses (agony, marked for death, rust, adramelech's touch, etc).
In terms of fire attack spells I have 1 Firestorm, 2 Fireball, 1 Devil's trident, and 2 flameblast. Again, still working on the amounts.
2). Right now, I have a relatively even amount of low/medium/high cost demons. Specifically, 3 Firebrand Imps, 3 Wildfire Imps, 4 Infernian Scourgers, 2 Flaming Hellions, 1 Blood Demon, Cereberus, Adramalech, and Sersiryx.
Similar to Dougla and yourself, I initially tried to go heavier on imps, but after a couple games of them getting slaughtered I had to rethink. With her ability card + her helm, the imps can do more damage than most lvl 1's, but like most lvl 1's they have no staying power.
Right now I guess my "go-to" creature is the infernian scourger. They're not my favorite thematically, but they have a decent body, do decent damage, and they trigger pentagram easily with their counterstrike. Triggering pentagram easily is important because it lets you transition into Flaming Hellions, which is her best creature, IMO.
3). I definitely prefer demons with flame attacks; they trigger her ability card. However, I think it's important to include both types because they function differently. I think it would be a mistake to include one group and not the other.