"Why not buy a 2 3/4" reamer and some cases,
2 3/4" cases will always be available, you can't
guarantee 3" will be IMHO."

Actually 3in is more common, very few 2 3/4NE made in recent years, just use more filler if you want to use the longer 3in shells in your chamber, this is not the best route, but do able.

I don't normally use shorter case's in long chambers , but I don't see it being a "real" problem, millions of 38Spl are shot in 357Mag chambers & 22 short were shot in 22LR, the worst was a carbon build up, in most case's, I'm sure there has been problems in the last 200 years with soft metals & black powder/Cordite, but if you clean your chambers & don't use it like a range/target rifle , I'm sure it would be Ok.

On the filler I have been using Dacron filler for years , other than it needs to be cleaned very well, as it has helped pit some of my bores, I think because the particle's trap moisture or some thing, it has been great .

I now use Backer Rod/Foam, far easy to use & more consistent , on this I read a report on pressure testing done with the 2 fillers & Dacron had inconsistent pressure & spikes, where foam was very consistent.

The Dacron results could be from loading form to as it is a little harder to be consistent, unless you weigh each Dacron ball ?