"Bottom line: would a 7k psi low pressure 2.75" shell be dangerous in a 2.5" chamber Lefever? I am reloading 2.5" shells anyway, but I would like to know if I can shoot some of the shells I already have. "

Bottom line is, no one can tell you.
There are many opinions and in some cases back of the barn tests but as an Engineer I use the correct shells for the gun I am shooting.
It's really up to you and at your own risk.-Dick