If I play the Will supporter that lets me choose heads or tails for my first coin flip of an attack, do I get to use Will's effect again if I use Glimwood Tangle stadium to reflip the coins?
Yes, if you use Glimwood Tangle, Will lets you choose heads/tails for the first coin of the reflip.
Source: TPCi Rules Team (2020-09-24)
Can I use both Victini's "Victory Star" Ability and Glimwood Tangle stadium to reflip coins from an attack twice?
No, you cannot. You can use either the ability or the stadium, but not both of them. As a general rule, you can't use more than one reflip effect at a time.
Source: TPCi Rules Team (2020-09-24)