If it's a player's dream to play at ever-higher levels, then he's going to have to become comfortable taking on all comers; and, many times, his toughest competition will come from his own teammates for playing time.
You can be assured that it's every college recruiter's dream to bring in a class full of "ringers" every year; just as it's every professional scout's dream to have every draft pick good be enough to one day make it to the majors. No playing spot should ever be considered "safe."
Sorry if this comes across as insensitive; but, living within the upper realms of any sport requires playing up to that level, and the only way to succeed is to consistently beat out the supposed "ringers" for playing time.
Yes and no. The MLB, NCAA, etc. all have rules about rosters. It's not like Virginia could go out and add players from other schools before they played Vanderbilt in the CWS.
Certainly, they can look to "upgrade" their roster by recruiting new players for the next season. But even with transfers, they would need to follow certain rules.