The little spacer that the Uberti "gunsmiths" put into mine looks kind of funky, but the trigger pull on mine is quite good. The mainspring tension seems about right for smooth cycling, although I did back off the lever and carrier springs a bit. I did a little smoothing on the internals, but didn't feel the need to work on the mainspring. If I had your gun, I'd be tempted to take a bit of copper or brass tubing, cut off little sections and put in in place a la mine. Unless you put so much pressure on as to damage the mainspring, it should be quick, inexpensive and reversible. It just might solve your problems by holding the spring in place without adding too much tension.
My understanding (although I may have been misinformed) is that the original springs were quite a bit thicker. The hook for the mainspring was copied, but the Uberti spring is thinner, making the hook a bit larger than it should be. No doubt someone with better knowledge will correct me here...
CC Griff