The way I'm reading this format is "Ignore the expanded G7 functionality of the move relearner, and the ability to inherit level-up moves regardless of their nominal level as long both parents have that same move to pass down. Build movesets out of nothing but egg moves and level-up moves that have a nominal level of 5 or lower. If the Pokemon you're using on the team could not have existed in its current evolutionary stage at level 5, back up a stage at a time until you find a stage it could have reached by that level, and the snapshot of level-up moves you get to choose from considers only that stage." (So for something like Greninja, who gets moves like Water Shuriken and Night Slash at level 0-1, you don't get to consider those moves; only Froakie is available that low so the only moves you get to choose from are Pound, Growl, Bubble, and egg moves. Evolution lines that don't have a representative that has ever been available as low as level 5, like Silvally and Naganadel, are presumably banned.)
Magnezone is not available at level 5, but it is available at level 6--that's only because it has to level up at least once from Magneton, who is available at level 5. However, the only way to get a level 6 Magnezone requires all of the following: hatch a Lickitung egg in VC GSC, use the time machine to send if back to Yellow, perform the in-game trade there for Lickitung, send that forward to Crystal, make another in-game trade for L5 Magneton, drop it into Pokemon Bank, withdraw in an Alola game, go to a magnetic area, and use a rare candy so that you stop exactly on 6 instead of overshooting to some higher level.
As a consequence of this, any such L5 Magneton or L6 Magnezone must have Analytic instead of any other ability, it cannot have spent any time in a G3/4/5/6 game, and since it was obtained in a trade, it won't have any egg moves (this last point obviously isn't relevant to Magneton, since its egg move list has always been empty, but it can be a complicating factor in a few other cases like Machamp). It also can't be shiny, since the in-game trade Magneton was locked to IV byres of 9666, but that's obviously less relevant. Now consider these points:
* Tri Attack is now available as a level 0 move for Magneton (and also, redundantly, level 1), but has never been available to Magnemite in any game
* Metal Sound was given as a level 1 move for Magneton (as well as Magnemite) in G3/4/5, but ever since 6 it's been pushed back to later levels only
With that established, do you agree with either of these legality checks?
* "If Magnezone wants to run Magnet Pull or Sturdy, that means it can't be the VC transfer, and crucially it cannot possibly have existed at the 'Level 5 (or lower) Magneton' stage. Thus, the operative evolutionary stage we should be looking at for legality purposes in this case is actually Magnemite, and that pool doesn't include Tri Attack, so Tri Attack Magnezone should not be allowed to exist in this format unless it also has Analytic."
* "If someone does want to use Tri Attack Magnezone, they need the evolutionary check to stop rolling back as soon as it considers Magneton stage. Metal Sound does appear as a level 1 move for Magneton in some games, but only in games where level 5 Magneton cannot legally exist, so Tri Attack Magnezone should not be able to have Metal Sound."
Not that I think it's a particularly good format anyway, but these are the kinds of questions you have to consider when you're determining your basis for legal movepool on grounds like that.
