While Nemesis had many flaws, the OP only wants to know if the movie would have been better without the Remans, so I'll attempt to answer that question.
First of all, I don't think the problem was the existence of the Remans per se. For decades we all knew of the existence of Remus (or Romii, or RomII depending on your take of the Neutral Zone diagram in "Balance of Terror"), but previous fandom simply believed that this second planet was also populated by Romulans as well. That's certainly what the early Diane Duane novels implied. Since there was absolutely no canon information up to that point about the Remans or Remus, if someone wanted to come along and reinterpret the Remans as being some kind of Romulan lower-class slave race that does all the grunt work in the Empire, that would have been fine with me.
The problem was the actual portrayal of the Remans in the movie. Contrary to KingDaniel's assertion that he thinks that space vampires are cool, I thought the Remans were not cool. They were basically Nosferatu rejects with costumes straight out of a Tim Burton Batman movie. They looked like vampires for no other reason than that they were supposed to be "EEEVVVIIILLLLL!!!!!" They were complete cardboard villains, and even the great Ron Perlman was completely wasted in this film. I think that at some point between the idea for this movie and what was eventually filmed, we the audience were supposed to feel sorry for these poor creatures because they were enslaved, but nowhere did I feel the least bit of sympathy for them for the reasons stated above. And to top it all off, the film didn't even have the courtesy to make it clear about just what the Remans' origins were. Were they originally Vulcans like the Romulans? Were they aliens that were already living on Remus when the Romulans arrived 2,000 years ago? If they were originally Vulcans, how could they possibly have changed to look like they do in only 2,000 years? Why would they have developed fangs?
Nor did I ever feel that Shinzon really gave a crap about the Remans despite what he said about wanting to liberate them, but of course that has more to do with Shinzon's problems as a character than the Remans themselves.
I do think the Remans were necessary to the overall story, but that they really needed to be presented in a completely different way, and have the audience care more about them. But the film didn't do either of those things.
I agree with you about the Remans being presented differently, and about their overly monstrous appearance. But I do think that Shinzon cared about them. He seemed to have a father-son relationship with his Viceroy and his best scene to me was his talk with Picard about Reman liberation. I wish this had been expanded on more. It would've given Shinzon more depth, but NEM was bound and determined to make sure the audience knew he was evil, so they undercut his nice speech with Troi's mind rape, just to remind the audience that Shinzon wasn't sympathetic, as if his slow walk intro from the shadows wasn't a big enough clue.
I also disagree that the Remans were supposed to be seen as sympathetic. From their conception as vampiric-looking beings, they were supposed to be monsters, and the Federation or Picard evinced little sympathy for them (not like his standing with the Ba'ku). And the Remans were pretty two-dimensional and brutish. They were not supposed to be characters or a species that was meant to be understood. Even the Son'a got more development and a backstory (as you pointed out the Remans didn't, and I agree we should've learned more about them). This could've been done during the Shinzon-Picard dinner.
Ultimately I don't think the Remans were needed. Shinzon could've easily been a renegade Romulan captain. The only thing that would've been lost is the clone angle. Also the excuse to get Picard into the action is already provided by the Enterprise being the flagship. If the new Praetor wanted a meeting, they would likely send Picard anyway. There was no need for the Remans (since Shinzon wasn't developed enough to make him a liberator and what little was done for that was dashed by turning him into a genocidal rapist). And the B-4 story wasn't needed either. Heck, taking out the Remans could've been the linchpin to making the movie better overall since it would've perhaps erased the need for B-4. And you had Ron Pearlman already, he could've been the main villain, though I could see him more as a Klingon or Jem'Hadar.