"This spell is ended if the Caster attempts to cast The Oaken Throne again, or if the opponent removes a dice from their Magic Dice pool at the end of step 3 of any Magic Phase sequence (after Siphon the Veil)"
1) Oaken throne is type 'Caster' which means it only affects the caster. so not all your druids
2) Fountain of youth is an Attribute spell : it is automatically cast every time you successfully cast another druidism spell. So you can only recover/raise a wound if you cast a spell and it can happen more than once by turn if you cast multiple spells with your druids