I see your mom agreed to a different approach to training, so a big congrats to you!
If it hasn't been suggested yet, have you considered moving his ex-pen to your room, so when you can't watch him, it won't be your mom's responsibility? This is the setup I had after my roomie freaked out on me, I put Kaji in an expen in my room when I was gone and inside he had a bed, food, water, toys, and a piddle pad when I switched him over to piddle pads. His piddle pad was far far away from his food and sleeping area so this area was always kept clean. I had carpeting, so to protect the carpet, the entire pen sat on a plastic desk mat like this one,
StaplesŪ Economy Mat for Commercial and Low-Pile Carpets, Traditional Chairmat, 36" x 48" | StaplesŪ. I know they make long ones but they can get expensive, but two of these worked for me, or you could use a shower curtain. I like the chair mat better because they're heavy, which makes them sturdy and they won't move around.You could leave it as is, the plastic won't hurt your little guy, or you could put a blanket to make it comfy. I have Kaji's piddle pad on a tray like this one,
Premium Pad Training Tray - Training - Training & Behavior - PetSmart, so his rambunctious activity doesn't bunch up the pad and he always has a place to pee.
I know you already have a bunch of good suggestions, but I thought one more can't hurt. As long as he isn't hit, and giving you sad eyes anymore (that post made me cry) you should be ok. You don't want a dog that does submissive peeing. That's peeing because he's scared which will make your mom think he's still not trained, when in fact that is another issue all together. Kaji does that when he's in a big strange place with lots of strange noises. He freezes and pees. He also excitedly pees which is why I put his pad right at the door of his ex-pen. He always waited there for me to get him and when I reached him, he would pee right where he was, which was on top of the piddle pad, he got praised, and eventually learned the right thing by accident. I apologize if anything I said has already been said, I didn't read through all your posts. GOOD LUCK!