O44,
I believe that mid-year transfers can legally practice beginning in the fall. The one year sit out applies to competition. There probably are significant practical issues with mid year transfers, including meeting academic progress requirements. First year players who have signed a NLI would also need to negotiate a release.
I don't follow why the 35 man roster limitation should affect injuries. The 25 man travel roster limit hasn't changed, and my sense is that even before last year, a player who wasn't on the travel roster seldom played at home either.
If there has been an increase in injuries, I'd speculate that it is primarily caused by the shortened schedule.