Hey 
First question is easy. If you give final hit and kill enemy (doesn't matter with which demigod and in what state you are: dead, out of range, etc), you get gold. So yes, you can kill an enemy with poisoned blood when you are dead and get gold. It might not write in score that you got kill, but you'll get gold no doubt (and that is what is important, not the +1 in your kills score). However if you don't do a finishing hit, you might get an assisst, but no gold while you are dead.
Second question is not for me... but still. My opinion about skills is:
Bite - reduces armor, deals damage, heals you. Armor reduce is good in combo with anything dealing damage - ashkandor, minions, giants, towers, etc.
Mist - perfect skill to remove debuffs (spit, mark of betrayer, snipe tracker, etc), force enemy change target, force enemy to retreat when attacking with your minions while in miist, avoid skills like hammer, fireball, etc... 1 point is enough I think, but again - my opinon.
Mass Charm - well mass stun... good for relocking, stunning whole team, saving teammates, immobilizing enemy for your minion damage, scaring enemy, stunning minions (a lot longer time against minions than demigods).
Cannot remember name, but skill, which adds more nightwalkers for Erebus. By default he has 2 nightwalkers, but with 1 point, you can have 6. Then later ones add +2 each, but first gives +4, so... More damage per second + health shield against towers, demigods, etc.
Bat swarm - catch, escape, avoid skills, kill, etc. Very versatile skill.
Poisoned blood - small life regen (compare to Sedna's 1 point regen skills). And damaging skill, which works only after death. It is incredibly good if you are not using Uberfix and know what you're doing (facepalm to Uberfix not wanting to use players). Go die to towers and your poisoned blood will drop (no gold for enemies). Someone from your team gets low health while not damaged by enemy demigods in last ~10 seconds and drinks your blood. You get gold for killing your teammate. Sort of not fair, but maybe that would make players want to use Uberfix a bit more. With Uberfix I think it heals allies instead or something, so such thing won't work. Against great players tough to be walking low on hp to suicide like that also. But besides this, this skill is not so great in my opinion, so can be delayed for later levels. 10 level poisoned blood at least deals decent damage instead of low ~750 (I think) earlier. Also, it is possible to know if you are using poisoned blood or not at later levels if you don't upgrade it to level 2 at level 10 and you die. Or any player can just test it and very rarely later on fall for it. But having that in mind, you can not get this skill early on and other players will still be afraid to get your potions, thus letting you heal yourself if needed with your earlier potion.
There is also vampiric Aura, which is not bad (gives everyone around life steal aura), something after mass charm to increase cooldowns. So you need earlier skills to get those.
I'm not pointing at skills, as I don't really know which would be good for you, but giving basic ideas about skills, so depending on situation, you could take, what you think is needed then. For example what first skill do you take? Bite if against someone weak, mist if against something like ub or being someone, who receives more damage and is mainly assisting, etc. I play probably more a hybrid Erebus, as Bite + minions is tough power, but most skills are of an assassin type.
I am rarely playing Erebus, so all of the above might be not good/incorrect and is just my personal opinion.
