Should I 5* Soleil

I'd try and wait for a +Spd one, but if you feel like it, go ahead. Her base kit is dependant on high speed, so that's why I recommend waiting.

Don't spend 20k feathers on a unit with the worst possible bane... wait for a better one and use those feathers to inherit a QR3, Aether, or merge a unit to a higher level.... if you're worried about not having enough good units, I can assure you, that those will come on their own with time

if it's -spd, it's not worth it. The idea of her set is to double heroes having the ability of not receiving damage while attacking. If you aren't able to double enemies in combat, you're not taking full advantage of her.