I've pondered about these questions quite a bit myself.
1) This is purely speculation but I think what would happen is Ran witnessing Conan turning back to his original form firsthand in the finale's climax so coming up with a lie would probably be unnecessary. That said, it would expose all the other lies Shinichi had dumped her but with Ran, I don't think she'll mind much especially since it means that she's basically reunited with the one she has been waiting for. That reasoning is a bit shaky though and I'd much rather have Gosho make it more realistic by giving Ran a year or two to finally reconcile with Kudo.
2) This one's a bit tricky cause it's a lose-lose situation for Mouri. XD The disappearance of his 'skills' would probably be unexplained to him, most likely for comedic purposes with him still being cocky despite his lack of competence. But what I do see/want happening is that Mouri rejoins the police force following the urging of his wife and daughter (this means Eri being together with Kogoro again after a catharsis inducing case involving their relationship ofc) and it was shown that he's at least competent in being an officer so it would be sort of a happy ending for him.
3) One idea I have is that someone telling the Detective Boys, be it Ran/Ai/Agasa or Shinichi with Conan's voice, that 'Conan' had to go back to his hometown. The heartbroken DB would then be taken by Shinichi under his wing as his apprentices or he'd treat them as his own Baker Street Irregulars in the same fashion Sherlock Holmes did. This would be a nice payoff considering it ties well with Shinichi's 'Heisei Holmes' reputation and it allows the DB to finally revel in what they enjoy doing with some 'authority' overseeing them.
Everything's most likely going to be a happy ending for all the protagonists anyway even if that means some logic have to be sacrificed.