請教小白兔- once again

While doing H-1B transfer, can there be any employment gap between the old employer and the new employer? If yes, how many days are allowed?

Would it be possible to keep a part time/full time position with the old employer while taking a full-time job with the new employer? If yes, how to make it happen?

If it happens, can I I-485 based on the approved LC and I-140 with the old employer for the previous full-time job? The old employer is willing to say the position is still available.