EDIT: I read what you wrote again and I feel stupid. And I remembered I fell to the same trap and I was also puzzled. The devotion "locks" that get removed is only part of the tutorial. For the rest of the game, you just unlock options as devotion moves past certain lock signs. E.g. if you ask for sex, the area between current devotion and 2nd sign will flash yellow marking that you need to have devotion past the 2nd lock sign.
TL: DR lock sign are just markers, they are not removed.
The game is a mess. With no explanation and weird mechanics. Top it off with infinite softlocks. Anyway, I experimented a bit and here is my findings.
To answer your question, it depends on the girl you are training (cell or guest). Basically, the stress will raise lower as devotion is raised. Also, the more stressed she is the more stressed she gets, look at the multiplier next to stress (x2.5 or x1 or even x0.0).
Cell girls: Goblins and aggressive human-girl after defeating them in combat. You can tell if girl type is aggressive and not timid based on her responses and starts unfriendly (around 30 percent). Don't attack timid girls as they will run away and die in 2 hits.
For cell girls: Just keep training them. Who cares what they think, they are just prisoners

For guest girls:
- Once she is pissed, talk to her to calm her down or come back later.
- Try to do massage first to get her into "comfy" state. Comfy happens when all areas are green (need deep massage, 2nd option)
- Pay attention to current mood (top right) as it will determine her reception of you. If she is nervous, she will be more cautious and stress raises faster.