Tiarawhy.com and Mittsies.com. Tiarawhy has all of Mittsies work on their site as well as BfED and some other adult animations and comics. The only other adult pony game that comes to mind is Love Collector by dailevy on Patreon.
I haven't been able to save the game. I'll try my other browsers but IE doesn't let me save. EDIT: Firefox and Chrome both allow saving. IE and Edge don't.
I only did that much since you said that the second Applejack fight was much harder. The hardest stat to grind is speed since that can only increase in the Everfree or through leveling up. Strength and Intelligence are actually quite easy to grind at the farm and library respectively so you...
Well I definitely did a lot of grinding,(35 intelligence for two special attacks, 50 endurance, 50 attack, 27 speed, and level 9 as an Earth pony) but I only had to use 3 of my 16 or so Health Potions.
I can't remember which keys unlock which rooms but there are four keys total. One on floor one, one on floor two, and two on floor three. On floor three, there is one key inside and one key outside. I think that you might be missing the outside key.