If you mean that you and another person are apping in characters that were already in a relationship, then yes, that's fine! There's no need to write out the consenting happening every time if you wish to do smut - the important part is that both characters are clearly consenting and that neither one was pushed/tricked into having sex after a clear "no" is given or the inability to consent is established.

This would only really apply in situations that involved angry sex, makeup sex, etc. Also situations in which one character didn't explicitly say yes or no, but was fully coherent enough to refuse should they want to.

The best way to go about this would be to show that BOTH characters are pushing the encounter towards sex, and that there aren't any ambiguous situations where one character is leading things and the other is completely passive, uncertain, or outright says no at any point. The nice thing about RP is that you can write out thoughts and make intentions clear, so just be sure to do that!

It should also be okay if a intoxicated character is initiating the encounter, as long as it's clear both during and afterwards that one character was not taking advantage of the other.

The important thing to remember is that the purpose of the dub-con rule is to make sure that characters aren't in an environment where they have a lot of reason to feel threatened sexually, since affectionate physical contact is the main point of the game. If both characters are consenting parties in a healthy relationship (though yes, angry sex is still okay as long as both characters clearly consent), there shouldn't be any issues. If a specific situation ever comes up that you aren't sure about, though, you can always send us a quick message and ask if what you're doing is alright!

This answer got very long, but I hope that it addresses your concerns and that I didn't misunderstand any of your questions here.