So...I had a chance to get back to my club this afternoon, and brought the RV410 with me along with a box of Troy Slug Storm 2.5" .410 rounds.
I offered for a couple of range buddies to take some shots with these, but after seeing my previous burnt hulls in person with the 3" ones, they declined and would just watch.
I loaded the 2.5" slugs into the RV-TAC410 revolver shotgun and....
No issues.
Took a look at the spent hulls and nothing was melted. Now the slug hulls were black so I couldn't tell if there was any charring, but if it wasn't melted, then that seemed to have solved it. I will write to EGE and advise them and up to them if they decide to want to continue to advertise it as taking 3" shells...which is possible, but I can imagine someone else freaking out after seeing those melted and burnt hulls, without realizing what may have caused it.
That said, seems like using the 2.5" shells is a good compromise. I also felt significantly less gas come back at me on the right side cylinder gap (that I don't think is shielded). Definitely the 2.5" makes a huge difference in this gun.