About Sparaxis vs Twinleaf, that's really simple. It's just a question of Pursuit vs Combo Hit and Combo Hit is better in most cases. The only scenario of Sparaxis being better than Twinleaf I know is versus enemies with extremly high defense. Pursuit Damages seems not affected by defense (or at least, it ingore a great part of it) and Sparaxis can perform better than Twinleaf low damages and low Combo.
Sparaxis is still great and only fail because enemies has way too much HP (that's basically the same for all the Pursuit mechanic).
Also, 100% re-act is not overkill since re-act is naturally caped at 90%. 90% is great but it will always fail when you really need it

Pavonia is obviously the best choice from Rainbow Medal's 6* Exchange. Diamond Frost and Crepis are okay too (Crepis not that much but she is from Blossom Hill making her a good option for Bloss team).

another girl you should put on your wish list is Flowering Peach (Halloween). She is very versatile as support.