I thought so too. Would love it if they actually explained it.
I think that Rimmer's status, much like the end of Series VIII, is like the Klingon forehead issue from
Star Trek. We already know the IRL reasons for them, and any in-universe explanation for them is bound to be less fun than the endless fan speculation about them. (I'll still never quite forgive the 4th season of
Enterprise for that.)
Everything's relative. While it may not be perfect, it's a damn sight better than anything you can do with prosthetic makeup to de-age an actor.
I actually found the Michael Douglas de-aging in Ant-Man startlingly convincing. They made him just like he had in real life at that age. The Kurt Russell de-aging in GOTG was less convincing -- he did look younger, but he didn't look quite like Russell actually had at that age. And his voice had changed quite a bit too. Oh, there's also the de-aged Robert Downey, Jr. hologram in Civil War -- that was also quite convincing.
With Robert Downey Jr., it helps that, when he was younger, he really did have a certain unfinished plasticy look to his face, which does a great job of matching current CGI technology. Kurt Russell in
Guardians of the Galaxy 2 certainly looks a bit beefier than the actor really did in 1980 but it looks convincingly human and it gets the point across. Michael Douglas in
Ant-Man wasn't perfect but worked well enough for such a short scene. Certainly the technology has come a long way from what they did with Patrick Stewart & Ian McKellen in
X-Men: The Last Stand or the often unrecognizable Jeff Bridges in
Tron Legacy.
The actors have also talked of hopefully bringing Ace back in future.
Maybe he hasn't come back yet because they can't get the actor back. ... ...Oh, wait...
I hope that whenever they decide to end the show that they close it with some special, like maybe a series long arc about getting back to Earth. Naylor has certainly had a long enough time to think up a satisfactory conclusion when that time eventually arrives .
I doubt it. I think he's just making it up as he goes and, like a lot of writers, does his best work when he's working from desperation, not because he's got some grand idea planned out. I doubt the show could ever end on a better punchline than "Only the Good."
As for bringing back Holly, much as I love the character, I suspect that they just don't bring him back because they don't have enough good material for him to justify his full time presence. We saw the gradual decline of Holly's screentime during Seasons III-V. The whole reason why they wrote her out of the show in Season VI was because they didn't think it was fair to have Hattie Hayridge hanging around all the time if they were going to give her so little to do. And when they brought Holly back for Series VIII, the same problem occurred and his material petered out pretty quickly. He's just not a vital cog to the comedy machine since they already have Kryten to do all the technical stuff and Cat to do all the dumb jokes. (And given the age of the cast, they can do all their own senile jokes now.

)