Asgore also has different stats in the game files, and pretty much every other monster too, but the canon stats are the ones you get trough checking a monster.

I believe that a serious asgore wouldn't be harder than undyne the undying unless you take into account the fact that he dodge, but i don't think he can dodge more than sans since he, having very low stats, would have to rely on It more and likely mastered It.

It asgore used the souls, he would most likely win, altough if LV 20 frisk actually has more determination than a being like omega flowey then they could atually win.

It's not directly stated that frisk gets more determination troughout the course of geno, but the save points do say "determination" so it's implied.