I think the movie thing made sense timing-wise because Dave was all over MTV and every magazine and ET and the like. The dude was literally everywhere and he says he'd had other movie offers before (who knows). So up comes the movie deal and why not jump at it? Eddie was dicking around and not wanting to work so why not do this in the meantime? He even asked the bros to play on the record.
As far as him working with Vai and the others before April 1st - again, that's Mikey saying that. But he had to work with other guys for the EP so it wouldn't shock me to learn that when Ed and Al passed on doing the music for the movie that Dave immediately started looking for other guys and during that 9 month gap started working wih them. That would fit timing wise wih what Mikey said. But again, it doesn't really matter - just something bouncing around inside my head