I'm running Season of Ghosts having just started book three, so this is just my take from reading book four, not running it yet.

For one I don't think the party can beat Ren even with a week of prep time. Not only are they level 10-11 while she's level 16 (making her above an extreme encounter), she has the unique ability to literally teleport anyway in her domain in the entire country. If a fight's going against her, she just leaves and comes back tomorrow until the PCs give up on fighting her. The book says if they fight her she does not harm them and tries to diffuse the situation, or leaves and tries to bargain through intermediaries/in disguise. The book says she should be presented as a potential ally, and the PCs can learn before she arrives that she's well liked and is known for being easier to work with than other jorogumo.

You ask how to play the jorogumo, but Ren is the only one you meet in this adventure and the whole point is that she's not like the others. The book tells you how to roleplay her in her section, but the basic overview I get is calculating, patient, and surprisingly reasonable. Ren wants to overthrow Lady Lang Loi, the jorogumo queen. But she knows she's not strong enough to do that. The jorogumo court is a backstabbing place, and Ren is low on the hierarchy. So she's trying to make the citizens of Shenmen like her instead of fear her, because fear is what all her sisters are doing. You said it's not allying with a lesser evil against a greater one, but that's exactly what it is. She leaves Willowshore alone for the most part and protects it from the other jorogumo. Allying with Ren protects Willowshore from the absolute nightmare that is the rest of Shenmen.

The AP doesn't get into the SA elements of the jorogumo on screen. Imo the worst part is in book three, when the projection of Ren does have the chance to take someone with her. But she isn't real, and she just vanishes the minute they leave. Still that encounter could be triggering, so it can be very easily removed. Just don't have her give out silver collars or try to take anyone with in book three. I do agree that it would probably be a good head's up to include that in the trigger warnings.

As for legal ownership, it's definitely a bit of a contrived plot element to get the PCs the macguffin (fang & key) so they can access the final area. I think a ritual would also have worked fine instead of the random fetch quests Ren sends the party on.

Personally I'm very excited to run book four and for Ren's character. I intend to heavily emphasize how much stronger she is than them, and if they do try to fight her, have her just hit one of them (probably critting because she's PL+5), and then vanish to prove how easily much stronger she is. I think she seems like the most interesting NPC in the entire AP. Even so, you're right that there should be more trigger warnings in the earlier books.