Saw this while I was at work sorry for not getting back sooner I just got home.
1. T1 breaks happen for a particular chosen if you do an action that gives the potential to reach 10k damage by the end of the surrounding turns that corresponds to that particular action. (I hope that sounds right haha)
Say you surround a chosen for 5 or 6 six turns and use "grind". The description in the game says that grind is tied to two t1 vulnerabilities- Morality (HATE) and Dignity (EXPO)
When you use "grind" the game will check every turn to see if the damage has the potential to reach 10k damage, I could be wrong but on the top of my all head I believe it doesn't matter which as long as the circumstance damage can reach 10k damage but to be safe I will just say in either HATE or EXPO for now.
This check is based on four things mostly-
-The circumstance damage the chosen already has taken in on those areas.
-The multipliers currently active on that chosen in those areas and damage you are doing to her in those areas (this is where minor/core/significant vulnerablities also play a role since they determine initial weaknesses for a chosen to certain damage)
-And finally the remaining turns you are surrounding her.
If the game thinks you can reach 10k damage based on that information then it will trigger a t1 break and iirc the order might be based on either the damage you first chose to inflict on her or it will happen is in order from Morality (HATE) Innocence (PLEA) Confidence (INJU) and Dignity (EXPO). I will double check this later today and edit this based on what is true sorry I'm mostly going off the top of my head.
ie. you surround Reaper for 5 turns and she has a minor vulnerability in Morality (HATE), she already has 1000 HATE and multiplier for her hate is x16. Reaper will go through those four checks we talked about before and if it passes all 4 she will "realize" this and use a selfish action to stop it from getting that bad, triggering either t1 morality or t1 dignity and then the other one whichever one you haven't gotten yet.
Which leads me to try and answer your edited in question of what happened in your game.
If what you said was true and INJU t1 broke then that means her Confidence was broken and not Morality.
In that case it makes more sense since "caress" is tied to 2 vulnerabilities, Pleasure and also
Confidence.
tl;dr: the chosen checks every turn they are surrounded and its tied to circumstance damage
2. There are only two permanent relationship stages Unbreakable Friendship(friendly) and Bitter Enemies (rivalry)
3. This question is a bit more open-ended in nature so to try and answer as best as possible. . . Every vulnerabilty break makes a chosen weaker to that associated damage and if that additional weakness makes them easier to surround and longer, then attempt to break a core on them. If you can see it happening in your attempt you have reached where you wanted. If you reload the save a couple of times and it won't work even though you tried everything you can think of. Change your plan and see what you need to do to get it to work. this also might include changing who you're targeting at the moment.
4.Yes they do impact the relationship forming in chosen to relationships. And you can still get a rivalry in my experiences it just might be a bit harder. If you continue breaking core vulnerabilities on the chosen while not breaking the rest of minor ones on the prebroken chosen the relationship will be friendly initially but eventually the relationship will turn negative and basically give you even more EE once that happens.
5.If I'm understanding your question correctly then yes they do compare to each other and if the vulnerablities are the same level or the minor is higher than the core it will be a positive relationship event and if the core is higher than the minor that means the event will be negative. I answered a question dealing with relationship between chosen and gave a more indepth answer below. I will quote in case you want to look at it
6. going off the top of my head I will try and list all the stages of friendsip/rivalry
Bitter Enemies- The Relationship is at the worst possible level and will never get better. Will show up to help the other way later than any other relationship. -2
Rivals- They dislike each other and will want to see the other fail. The downtime events will be mostly them trying to outdo each other. They also will arrive later than usual to help them in battle. -1
Acquaintances - Neutral and where every relationship starts at. Once a relationship progress past this they will never return to this level. 0
Uncertain Friendship - Were rivals before and had some moments to bond (positive events) Can become rivals or progress in friendship depending on the next event.
Solid Friendship- Will arrive earlier to help each other and do more downtime events together.
Unbreakable Friendship- The best possible relationship between chosen and once they reach this it will never worsen. will show up way earlier than other relationship stage.
7. Yes every chosen has a core that correlates to another chosen's minor it all depends on how you planned out breaking them.
I hope I answered most of your questions I putting aside some free time before I start drawing again so I will double check what I said about the t1 to make sure they are accurate for you.
Other than that I hope your run goes well and you learn more about the game!