I'm not sure if anyone still cares, but I actually labbed some stuff on Peach's Castle (64) a few months back with regard to Charizard's Uthrow.
Basically what happens is when you use the throw, you (and the opponent) ascend a certain distance that reaches to a point just underneath the upper blast zone from where you initiated the throw. After that, you then come down again, descending by that same distance, but
not necessarily ending where you started:
- If the descent is blocked, you'll finish the throw where the descent is blocked. (What you'll usually be trying to do by landing on a higher platform.)
- If neither the ascent or descent are blocked, you'll travel up and down the same distance, ending up in the same position as where you started, even if the platform on which you initiated the throw has disappeared/moved out of the way. (The phenomenon described in the OP.)
- If the ascent is blocked but the descent isn't, you'll travel up into the vertical obstruction and get stuck on it before descending the distance between where you initiated the throw and the upper blast zone, ignoring the obstruction, resulting in you ending up lower than you started. You can achieve a double-KO suicide throw like this. (Not possible on any legal stage as it requires a moving platform underneath a solid roof, such as in Peach's Castle.)
The reason you can't use FLUDD to push Charizard during the first half of his Uthrow is because both Charizard and the character being grabbed become intangible during the ascent. However, you become tangible again during the descent, so it's probably possible to FLUDD it on its way down and cause it to land somewhere a bit higher in the air for an early KO. There's no way to force a suicide throw outside of the scenario mentioned above, though.
