Is the convex combination of two potential games a potential game?
NO.

The following two-player game, with e very close to 0, is a potential game:

(e,0)   (e,1)

(0,0)   (0,e)

Taking the 8 equivalent games (switching the roles of the players / rows / columns), then a convex combination of them gets you as close as you want to any game with positive utilities, like to matching pennies with a positive constant added to each utility.  But that is not a potential game.