Did you not read my post from last night? While I didn't flat out state it then, I will now:
Your definition of an invalid variant is garbage, junk, meaningless, without merit.
I will say that again so that it clear:
Your definition of an invalid variant is worthless.
A variant of a game is either that or it’s not. If it’s a spin-off of another game, it’s a variant, if it’s not a spin-off of another game, then it’s not a variant of that game. A GAME (whether it’s a variant of another or not) may be valid or invalid depending of whether or not it’s winnable. Go-Moku played on a 4x4 grid would be invalid. There’s not enough room on the board to place five stones in a line and therefore, no way to win. Thus the game is invalid. But a variant can’t be declared invalid just because of how it stacks up to another game.
Fun-pente is a game and it’s a variant of pente. It’s valid, because it’s winable. The fact that one side enjoys an advantage is irrelevant.