3)
a) yes you can kill yourself with geno if you don't have much hp, you are @-like, not p-like, you in other category of monsters
b) example: you have total of 320hp, you kill with geno 5 monster at 72lvl so for each monster you loose from 1 to 23hp avarege 12. So you will around 60hp, so you still have 260, if server is configured, you will get penalty for using genocide spell, and you will loose half or ramaining hp, so you will have 130hp at the end. but i don't know how if you get penalty on main server.
c) dunno
4) best option don't fight them. if you have some kind of detection just don't go near them. You don't need to kill everything
but if you really want, you can just blast them from far away with regular spell, or use some rod/staff which cause damage.
You can be lucky and be able to kill them when near, but i will not risk that.
5)
a) kill dragonriders, or wyrms
c) you can't control effect, picked at random
