You are correct that the spring wears after some use and your method for fixing it would work but there is a much easier fix.
If you carefully remove the circuit board you will notice that there is an upper and lower board. The upper board is only supported on one side. The side directly under the left micro switch has no support at all. When you press down the board flexes a bit which causes a rebounding action of the board. This "bouncing" effect is what causes the multiple click registrations. When the micro switch is new the inner spring has enough tension to resist this bounce but after some wear it can no longer make up for it. Simply adding some rigid spacers between the boards on the left side will firm up the upper board removing the rebounding potential and make the micro switches base solid.
I took a couple of plastic zip ties and cut the bulky ends off to use as spacers. You could use any rigid non-conducting material as a spacer but I found that my zip tie ends were just the right size to fit snuggly in the gap between the boards. A little hot glue may be required to hold the spacers in place but mine fit tightly enough that no glue was needed. After putting in my new spacers the mouse works perfectly once again without the need to disassemble and repair the micro switch.
I followed Stitch140 example and signed up to thank u for your easy and quick help. I fixed my steelseries ikari optical with some plastic zip ties too, and it worked perfectly fine.
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