Regarding your TCs, run them all at 11 for feeders, for the sake of sanity don’t run anything that gives you heroes you MAY want when what you lack are mats.
For your hero dilemma, do you have stiff competition for emblems or will any one of them be able to have emblems when maxed?
Triton IS a bit tilted towards attack stats and the emblems make a huge difference (that said, I went all attack because he will never be on my defense team and over 800 is very nice). The boosted healing is an added benefit that will certainly help you in tough battles but I wouldn’t assume it will. He packs a decent punch so can help with the killing.
Sonya is helpful and I feel she is a debuffer that does a bit of damage (firm slap). Since my 4* healers were Mel and Sabina I didn’t use her much when 4* heroes were my mainstays (Rigard joined my team very, very late (for major healers), Kiril and Boldtusk were also late joiners so only were maxed for war depth and class challenges, not main raiding team). Sonya’s attack stat is low so my assumption is with her you need to be prepared for a war of attrition, not nuking. Do you have both healers and strong snipers to surround her?
Triton is more nuking since you can never be sure he’ll be there long (although even dead you still benefit from his excellent tile damage).
My belief is that you will be better served by Triton at this moment. A fast mana, glass cannon with a niche secondary special.
I discount Vela for you now because it seems from what you wrote that you are still building up depth, working on 5* too soon leaves you with a shallow bench and best to max the workhorses of a team, the 4* first.
That said, emblemed Vela shows up frequently in upper Diamond, with emblems she does cause problems.