Wildfire’s powers are still fire based, yeah, but I’m thinking that the trip through time did something to alter them enough that younger!Ace can still eat the Mira Mira no Mi. I might do something with soul fire, or star fire as a work around. Maybe claim that what older!Ace ate was never actually the Mira Mira and was a different Devil Fruit entirely, which is what led to him skipping back in time.
Yes, in fact Riot is going to track down the Yami Yami no Mi with the intent of using it to bait a trap for Teach. Only ASL end up reuniting at the same time Riot is absconding with the Fruit. So there proceeds to be shenanigans that result in Sabo eating the Devil Fruit in question.
That puts a considerable wrench in Riot’s plans to kill Teach dead, to say the least.