In the thread linked above, CloudDriver notes that the spring under that retainer is a stiff little biter that Velvac assembles with a press. Then, UN-Fortunately, they try to just stake that pot metal to the retainer and hold back all that spring pressure.
What I suggested was look into setting a pipe nipple or piece of threaded tubing (try a lamp store) into the mounting bracket with epoxy. Then use a nut and washer to compress the spring and hold the repaired mirror arm in place.
This is only a thought process. I haven't repaired one of these, only seen the photos. If it worked out that the spring requires a lot of travel to compress into position, put the nipple or tubing in place with additional length and cut the excess off after assembly.