If two players are on PC, and one is on mobile in a Trio Speedrun (example), would the run be verifiable or do all players have to be on the same platform? And if it is verifiable, would the platform to be chosen be my platform (PC) or the majority platform?

Yes, but you would still need all POVs. So your mobile friend would have to get a mobile recorder and be able to upload it to Youtube. (Which should be possible because of the hundreds of thousands of mobile recorders)