Go with light, it will be much stronger in the long run if you have fluffy.
I'm with Slashley, Michael and Tish are the ones you want. LT is good, but she will be obsolete soon, like, in a matter of days when Lugh comes out (not on current mtix though, which completely messed up my planning x_x) and when Iris comes out a couple months from now.
1. Correct, base is 80% and each light sub eido adds 4%. It is advisable to go for stats and useful actives instead of trying to get fluffy to 100% though. The incremental gain in elemental isn't worth sacrificing base stats for unless you have a really strong grid already.
2. Drop Forseti and use Diana unless you're going against debuff resistant stuff, in which case I would still drop Forseti and use Durga if you are not AAB-ing (Durga is fast but one of her abilities is a 2-turn rampage, which requires planning.) If you hate Durga 'cos of her RNG skills, use Sukunahikona for debuff resistant stuff. I only use Forseti when I can't Ex BP for whatever reason or she's my only way to hit 50% atk down if I needed that for some reason. Usually for GO where I have to use non-SSR or soul used to deal with an annoying mechanic or something.