Re using the old line set typically doesn’t hurt anything as long as the pipe diameters are correct.
Line sets have not changed since the introduction of 410-A.
Of course the right approach is to change the line set but sometimes it is just not practical. If the line set is flushed, the appropriate size and pressure tested I don’t see the problem.
Mineral oil uses Chlorine to attatch itself to the refrigerant (410-A does not). The Cholrine provides the attraction for Mineral oil to return back to the compressor.
Since there is no Chlorine in 410-A there is nothing for the oil to adhere to. The oil (mineral) tends to get pumped into the evaporator and coats the inside of the coil, thus providing bad transfer.
Mixing oils doesn’t hurt the machine but creates capacity loss due to bad heat transfer.
Typically you can mix up to about 5% of oil before exhibiting any issues.
So if you replace the indoor coil, the lines have no traps (or storage of oil) and flush the lines I cant see how you would contaminate the oil enough to experience capacity loss.
Just my two cents…