I think better is switch Domitia with Joon, so that if mana full at the same times, it will at least dispell attacker first before doing other damage, skill will cast from left to right.
Domitia - Magni - Elena - Evelyn - Joon
Richard I think is better tank candidate, also Magni and Elena conflict emblem class you can try other option:
Domitia - Elena - Richard - Evelyn - Joon
After Sartana costume maxxed:
Evelyn - Elena - Richard - Joon - Sartana-C
Evelyn should definitely be on the left wing, if she’s in your defense. (Since on defense, the system fires left to right and you want her debuff to fire before anything else.) With no changes to personnel, I would say: Evelyn - Magni - Elena - Domitia - Joon
I have a slight preference for Sartana over Domitia due to her speed, and, in your case, Domitia’s debuff is a little less handy given Evelyn.
I would make a minor change to @jinbatsu’s suggestion by swapping Elena and Evelynn (there’s a great post about where to position your heroes). Based on that, I would go:
Domitia - Evelynn - Richard - Elena - Joon
Note that the guide would not put Domitia in the wing. Elena is not a great tank. Horghall is your truest tank, but I would prefer Evelyn on defense over him (assuming you want a rainbow defense, which is a good idea, generally.)
You may also consider swapping Malosi in for Joon when the former is leveled up.
That’s not how costume works. You can level any version of the hero. The thing is, you cannot ascend the costume version to a particular tier if the original version has not yet ascended to. That means if the original version is stuck on the first tier, whether from level 1 to 30/40/50 (for 3, 4 and 5 star heroes, respectively), the costume version can only max the level of that same tier. Once the original version is ascended to the second tier, only then can the costume version be ascended to that same tier and can get to 2/40, 2/50 and 2/60 for 3, 4 and 5 star heroes, respectively, even if the original version is stuck at 2/01.
Again, the costume version can only be ascended to the same tier which the original version is ascended to, but can be leveled and maxed at that specific tier where he/she is currently in.