Best choice this MT would probably be Amaterasu.

Important notes:
1. I refuse to use Uriel because of Waifu reasons.
2. Next MT Im probably picking up Vahagn, now having Amaterasu would mean Im more versatile.
Amaterasu is already falling out of the meta if Uriel is not an option Fire Amon would be best.

1. Would it be worth to spend eidolon orbs to buff Ctugha?
No, Elemental attack>Character Attack get Andramalech instead

2. Should I save my eidolon orbs for Kaiser?
Yes but you should get Tiamat(stat block) and a good primary eido(Andra,Amaru,Icarus) for every element for which you dont have one yet first.