I don't know if maybe it's the specific order that I did it in, but I killed Yngling first and then went and killed Orvas, and when I talk to Crassius Curio, he says that it's a shame they had to die, but I still cannot be named Hortator.
Additionally, when I look at my map, I still see the 2 arrows remaining on both St. Olms and on Dren Manor.

#1
Extra Info:
http://www.uesp.net/wiki/Morrowind:Hlaalu_Hortator
Yngling Half-Troll
Yngling Half-Troll is in Vivec, St. Olms Canton Plaza, Yngling Manor. He'll ask for 1000 gold if you're a member, or 2000 otherwise. An alternate route is to actually kill Yngling, which is quite alright with the other council members if you talk to them about it (such as Dram Bero). A good opportunity to kill him will provide itself during the Yngling's Ledger quest - refusing to give him the ledger will provoke an attack.
Taking Care of Orvas Dren
In order to convince Velanda and Nevena, you must take care of Orvas Dren first, which may mean a number of things. You can just kill him but he is well armed and has a few good bodyguards. You can blackmail him if you obtain some papers locked in his basement (his bodyguard has the key: another one lies on a shelf near Dren himself, if you have a good enough combination of Chameleon, Invisibility and Sneaking skill to steal it.) Or, if you can raise his disposition high enough, you can talk to him about protecting Morrowind, resulting in Dren giving you his vote for Hortator. If you're in House Hlaalu and you've done the quest to take over the Camonna Tong, Dren should just give it up and say he'll use his influence on his two puppet councilors. Of course, if you completed that quest by killing him and breaking the link between Hlaalu and the Camonnas, rather than taking them over, there won't be a problem...
Orvas lives on the Dren Plantation, a large sized manor just north of Vivec. If your disposition is low when you try talking to him, he may attack you. If you're lucky you will be able to attack and kill him without alerting the other guards in the house.