I've finally gotten hold of my daughter's DIMMs, and they work a treat...no noticeable slowdown in either startup or response time.
Happily, I've got 2 or 3 days left in my Crucial window and was able to set up a return for a refund. Phew! Great timing.
Considering how many times I've handled DIMMs with no ill effects, I'm certain that Rita's original DIMMs died on their own, but, of course...