The loose rib in front of the forend lug may indicate a loose forend lug as well.
A loose lug will push against the rib especially when the gun is fired but also when the forend is latched in place.
Yours doesn't really look loose and appears to be the reinforced lug.
Give the lug a few taps with a hammer back and forth and watch for any movement. It may also lift upward slightly when doing so.
Sometimes the rib will resist loosening that much and only a short section right in front of the lug comes free and pops up like this one did.
As for a trip spring,,about any small coil spring of diameter and length that will fit and has enough power to do the job is fine. It doesn't need to be AHF old stock.
The trigger I would bend under red heat.
Just my experience from a long time ago when I had a similar LCS trigger that I bent back cold and it broke.
Maybe this one would bend cold just fine and all the other ones like it as well. But have always used heat since my experience. They say to learn from your experiences...
..and those American Classic SxS's are certainly rough on the inside aren't they!