A bit late now..... But...
To put this simply as possible, why did you feel the need to even touch the strut upper mounts? They don't even need to be loosened or disturbed in order to get the lower wishbone out and back in. Did you follow a tutorial written on here by another member who recommended it as a process?
Go to www.alfaworkshop.co.uk
and look at the how to tutorials ....
There is a very easy method on there... Made by Jamie and considering that they do this job thousands of times a year, they know the quickest and simplest method to do this job. Complete with photos. I would confidently think, they know what they are doing by now
It disturbs me when I see threads where members are actually removing struts, driveshafts and brake calipers from hubs in order to do such a simple job.... It basically adds further complications on removal and prevents you using the stiffness and security of the hub to the strut to your advantage when fitting and removing the wishbone.
There is always an easy way and a hard way to do anything on an Alfa
Not a safe process, but if you are really stuck with the need to compress a spring... Large thick cable ties and a small enough G clamp using the tightness of the cable ties to take up the slack on each side of the spring while the clamp is compressing just a couple of coils that you can get between. Once settled in position, cut the ties.
I will leave anyone reading this thread one last piece of advice.
When working on any used car the rule of thumb is... The more items you remove, the more chance there is to screw the job up. Things like rounded or seized nuts and bolts can put pay to the best laid plans... So... Keep it simple, less is more and... Be prepared with the right tool for the right job