Half agree with
Dealbreaker, because appears unchanged, but in reality, the change is there! What is the last time we see MC and Rina act as a couple, a normal couple? Doing things that don't involve sex? And even in the sex part, it's been a while since the MC and Rina had actual sex, because lately all the MC gets is half-asses handjobs! And, in the last events, not even this! I think that they are already in a path without return. They'll not be able to come back to their normal lives, even if the things with Kenji ends!
And that because she has been growing distant from him, intimally and emotionally! All the thigs she does, she does with Kenji, and if the MC is missing, it's all OK as long as Kenji is there! But not only this, because the MC is also too far gone! He can't avoid the stimulation of seeing his wife fucking with other guys! Even if it hurts him hard! And it does! We can see it in those last events, the one in the resort, the one in the beach hut, and now this one in the hotel in another town! The feeling of helplessness, loneliness and abandonment are starting to take a toll on him, but he can't stop! It's like a drug to him!
And this helps respond about the "only NTS" or "also NTR" paths. I think that this is not NTS anymore, but it's converting into NTR. You see that beach event? It was supposed to be a relaxing day in the beach in family, but turned out to be an imprudent fuckfest (for Rina and Kenji), and a half-ass cuckold session for the MC, independently of previous agreement!
And now, this event in another town! The plan was to have a relaxing day, enjoying a trip in familiar scenarios, a nice couple short vacation! But the "devoted wife" already had other plans for this "vacation", which involved in fucking Kenji in front of the MC, saying it was "a surprise"! The unforeseen event that didn't let the MC go on the trip only made it easier to go along with her plan! But what would a really devoted, loving, wife would do? If her husband can't go on the trip, cancel the damned trip and go another day! She said herself that there wasn't a need to make the trip on that day! But she decided to go anyway, taking advantage of the setback!
This shows that things are starting, and accelerating, to progress without the consense, hell even without the knowledge, of the MC! Only remember that comment of Rina, where she said that she "wasn't going to do anything, unless it feels right at the time"! It's a big change from the "I will not do anything, unless you want it" from the beginning! MC interpretated as "when he gives the greenlight", but this is just trying to cover the sun with a sieve, because it's clear that this is not the case!
What I mean is: things are not entirelly NTR, because Rina didn't completely abandoned the MC for Kenji, but the emotional withdrawal already began! This is how almost every NTR start! The feelings the taken partner have for the stolen one start to fade, being replaced by immediate joy brought by sexual pleasure! The process continue to a point that there's nothing left! Good thing for the MC that Kenji is a nice kid, and didn't try some domination shit with Rina, because she is in the point in which this would be VERY effective! But nonetheless, the consideration for the well-being, true happiness and even the will of the MC is starting to not be a priority anymore, what shows that distantiation of feelings between the couple!
And, in another hand, by the same reasons, it's pretty clear it's not NTS anymore! After all, NTS is always previously consented!
. No matter what the explanation is, it's the same result 
