Play Overlord H (the old completed one) to get an idea of all characters available, it's quicker to ask if specific characters will be available.
Currently, as of R2, there are only Shalltear and Albedo. R3 will have bigger scenes for Albedo, and introduce Solution (she'll have roughly the same amount of scenes Shalltear/Albedo currently do in R2).
Mare is male, male characters are not available in this game. Aura will naturally age because the story spans years, but her starting age is already much older than 76 for narrative purposes: In D&D, a 18 years old elf and and a 18 years old human are physically identical, so you're right in saying it's unnecessary - However, in terms of elf society, hitting 100 years old is a big deal and I'll be writing a scene for that, which is how Aura will be introduced in the protagonist's harem.
This game will: https://www.patreon.com/winterfire/posts/i-am-overlord-165464211
All my games are pre-planned so it's not a matter of changing my mind, I just follow the plan until the project is complete