So, I noticed Mountain's Stoutness has this line:
"If you also have the Toughness feat, the Hit Points gained from each source are cumulative."
Does this mean that Mountain's Stoutness and Toughness don't stack with the Gray Maiden feat Unbreakable? Or do all three stack with each other, and this line just causes confusion?
All 3 stack with each other.
very often they put reminding text into things, not to make them special, but to help you have all info in one place.
PF1 example is saying that a monk's unarmed strike doesn't have to be a punch.
That said, the dying saving throw changes from Mountain Stoutness and Toughness stack with each other, but NOT the Mercy feat the Paladin has (forget the exact name).