Since the problem seems to be magazine related, the change almost has to be with the magazine...and since the magazine(s) is/are undamaged...the spring would be the most likely culprit, since it's the part most subject to change with use. The next suspect would be the follower angle.
As to why it feeds when you push up on the magazine...*shrug*...who knows.
Try the EGW mag catch and see if it cures it. If it does, be happy.
This is the best advice you have been given. Again when dealing with 1911s always follow Occam's razor.
I know you stated some of the mags are new. Where did you purchase them. From time to time there have been batches of fake Colt mags with the "M" mark on them. These fakes where not made by Metalform and were often found on the gun show and secondary market. IIRC all 7 round Colt mags have hybrid feeding lips. Fakes have waddcutter feed lips.
Have you tested the gun with mags that are know to be 100% in another working 1911.