If you want moderate reform you can forget about Keranni. She'll never vote for it. Unless that's changed in one of the patches.
Nabith needs 5 influence to vote for moderate reform. Your starting influence is somewhere between 0 and 5 depending on your different decisions to support the church in Yhilin and the Ivalan faith in general. You can get up to 4 extra points of influence during the council events themselves (Technically you can get up to 8 extra points, but the other points can't be done without losing one of the other high priestesses). That means you need at least a 1 at the start to get her vote. (2 if haven't been talking to orcent a lot throughout the game).
You get +1 for arguing for "church unity" during the excommunications meeting.
You get +1 for appealing to Nabith during the last vote of day 2.
You get +1 just for choosing medium reform (+2 for light reform).
You can potentially get +1 for having orcent speak, depending on how well versed orcent is in religious issues (which depends on how many different places you've spoken with orcent in the past between bar visits and talking to him in base).
There's also a +3 for arguing hard for nabith's support, but doing so will anger Betricia and lose her vote.
Esmera will require you to agree to her deal on the first day and get 8 influence with her. Her starting influence depends mostly on how much you've helped Aram in the past and your economic strength, but you can also get 1 extra starting point for having a lot of control of Farnan earlier on. Starting influence ranges from -1 to 8. You'll get 2 extra points for keeping your deal with her. You can get up to 2 more points during the council. That means you need at least 4 points with her at the beginning to get her vote.
You get +1 for appealing to Esmera during the last vote of day 2.
You get +1 for having orcent speak. She's much easier to impress with orcent, and you don't need to have spoken to him as much.
All of these characters require you to talk to them before they cast their final vote to convince them to vote for the reform.
Edit: It looks like you can actually get Nabith's support for medium reform with only 3 influence if you can convince the succubus band to play for the council. You have to have convinced them to travel in the "House of Love" in Helvana (costs 100000 pron) and then talked to them in the guardhouse by the entrance to Ardford and convince them to come play at Gawnfall when you visit Ardford during the council.