Is restarting the game, for example to mash through sans dialogue allowed?

yeah

If it isnt explicitly mentioned in the rules, its allowed

Thats actually really helpfull thanks

Alternatively, if it's done in the WR video, then it is allowed.

Reloading the game isn't a glitch, after all. 'Glitchless' simply means "Don't exploit errors in the game code that Toby did not intend", essentially. Therefore it covers all the actual GLITCHES that can occur in gameplay. Reloading the game after a save flag is simply exploiting the way the game's save system works.