Question: Why is the man's groin blurred out in the fight-scene at the Japanese sauna? It's an R-rated movie (or unrated, depending on the cut of the film you watch), so it's not like they couldn't have shown it. The best I can figure is that it's a reference to the fact that genitals are blurred out in Japanese media due to "decency" laws. Is that what the joke was?
Question: How come Deadpool is aware of the fourth wall, but none of the other characters in the X-Men movies are?
Answer: That's one of his superpowers, "Comic Awareness" (sometimes called 4th Wall or Medium Awareness). This is something that Deadpool has in the comics and was incorporated into the films. Other than being a framing tool and to be funny, I don't know if the creator, Fabian Nicieza, or any writers have said why they gave Deadpool that power.
Question: In the first timeline ending, Russell (Firefist) is not convinced or changed by Deadpool's pleading; in fact, he casts Deadpool aside. Cable then lunges for the semi-auto handgun and takes his last shot, which is intercepted by Deadpool in his left chest (a fatal wound). Seemingly, the only thing that really changed Russell's mind was Deadpool's actual death scene, as Deadpool rambled on with his farewells and gradually faded away. But, in the alternate ending, Cable goes back in time a few minutes and uses an arcade token to stop the bullet that killed Deadpool; thus, Deadpool doesn't die from the gunshot and Russell doesn't react to Deadpool's farewells (that never happened). So, what event changed Russell's mind the second time, if not Deadpool's actual death?
Answer: His change of heart came from Deadpool's sacrifice. In the second timeline, Cable saves Deadpool, but Deadpool had no way of knowing. Firefist still has a change of heart because Deadpool was willing to sacrifice himself, even though he was ultimately saved by someone else.
Now, I can accept that in theory, except that Russell repeatedly saw Deadpool putting his ass on the line to rescue Russell. I mean, Russell knew from the very beginning that Deadpool could have killed him (but chose not to) and took some severe ass-beatings on Russell's behalf. Russell was really, really hard-boiled, and I'm not seeing that Deadpool almost getting killed as enough impetus to change Russell's heart. It seems (to me, anyway) it was Deadpool's actual death that changed Russell, such that a mere deflected bullet would not have the same effect.
Deadpool saving Russell in the film is what made Russell think that they were friends. When Deadpool tells Russell that they aren't friends, he remains hostile toward Deadpool, not believing him when he later admits to caring for Russell. At this point Russell is too far gone and will kill. However, it's only when Deadpool takes a bullet for Russell, fully intending to die in both timelines, that Russell sees that Deadpool really does care about him, and would have died to save him.
Answer: In science fiction there are two different ideas regarding time travel. In one, the timeline is fixed, so a person who goes back in time does what already happened in their own past, like in The Time Traveler's Wife - however, this is where the grandfather paradox comes in. The other theory as express in the Back to the Future series is the past can be changed and in so doing change the future for the person who changed it. Deadpool 2 follows the second concept, so Firefist doesn't need any motivation to go back the second time and in fact doesn't go back a second time since the timeline is already corrected and that doesn't present a contradiction.
It presents the contradiction that Deadpool's actual death broke Firefist's cold heart the first time; but the second time Deadpool doesn't die, so Firefist should have no change of heart.
"Except that Russell repeatedly saw Deadpool putting his ass on the line to rescue Russell." Yes, but there's a huge difference between risking your life to save someone and directly sacrificing yourself. Doing something that could get you killed and doing something that will definitely get you killed are entirely different. You may not agree with the change of heart, but that's how it's presented.
Answer: The Firefist the second time around is the one from the first who jumped back in time retaining those memories, and therefore remembers the events from the first time, just like he remembers to place the token to stop the bullet and remembers that he used the device a second time. He doesn't need to experience the death twice to have the change of heart remain.
Answer: It is a possibility. In Japan, the showing of a person's genital area and/or pubic hair is considered obscene.