Even given the unusual circumstances, it seems odd to me to knock him back down to Commander, even if he goes back to being first officer.
I always thought that allowing Riker to remain first officer on the Enterprise was his reward for saving Earth while in command in BOBW. I'm sure many people thought it was a good idea for him to be there with Picard just in case.
But after 15 years, Starfleet was fed up and the captain's rank offered to him in Nemesis was the absolutely last time they would do it.
From what I understand, Riker had a choice as to whether to become a captain or to remain first officer to Picard as commander Shelby stated by saying something to the effect of "you'll have your choice of command on any ship" or something like that, but he stated that he wanted to remain with the enterprise. I also always got the vibe that part of the reason why Riker never took the captain's chair is because he wanted the enterprise again as stated in Generations
where he thought he'd "get a shot at the this old chair" referencing the enterprise. I do believe that him assuming command of the titan was sort of a last time the captain's chair would be offered to him. In the Nemesis
script that never aired, Picard and Riker spoke about how the Enterprise will one day be his and that the titan is a fine and beautiful ship...but of course since it never made it to air, it's not canon.