Whether I will be protected with a speedrun if it is done on a weak device

And Can I Get Into The Speedrun Leaderboard

Hi ! Not sure I fully understand the question but as only the in-game-time counts for Mad Dex speedrun, having a laggy phone doesn't impact the final time you have. So yes you can get in the leaderboard

It does impacts the timer for Full Game run since it's timed RTA, but it doesn't matter for ILs runs (you can even submit a screenshot for ILs runs if it's not a wr)

Well TASes are done with slowdown, which just reduces the games' fps, not the timer which is pretty much the same thing, so I would consider adding a rule where your fps can't go lower than X fps. We can set it to something like 45 FPS to reduce the amount of (kinda) TASsed legit records.