;tldr I'm an engineer, like Harold74. I agree with him. I'd not be confident about that stay. Then again, I weigh near 250lb (a bit less), so I should be more circumspect than the average bear.
Your tape measure assessment shows about a 1/16 deformation inward. To be a bit more definitive, you might use a straightedge to check the straightness of the tube (it's tapered, so use a straightedge on each seat stay and compare). If that stay is really bent I'd really be a lot more suspicious that it could collapse (the seat stay is in compression, so buckling is the failure mode). Anyway, put a straightedge on the stay on the side opposite the dent. Compare to the straightness on the other stay, measured in the same (that is, mirror image) way. If the damaged stay shows more of a bend than the other stay, I'd really be more likely to avoid riding the thing, at least hard.
It the stays are generally the same straightess, you could apply a fix. It would be ugly but you could "splint" the thing with slightly larger tubing, where the splint id is the size of the frame od. Sand the paint off the stay near the deformation, slit a piece of carbon fiber tube, epoxy it on and apply band clamps to let the epoxy cure.
The anecdotes about steel frames don't really apply much to aluminum. Different beast. Aluminum is much more susceptible to work-hardening so you can't just "bend it back". Aluminum is also difficult because while you can repair a tube by welding in a new tube, you have to heat treat the welds (there is some research into welding fillers that don't require this but...) and this generally means you HT the whole frame, which requires new paint or powdercoat. And you have a hydroformed frame, so it's not just replacing tubing...
I suspect that if you check with Trek, they will take the lawyer's position "Oh! That's unsafe! There's no way you should ride that frame!". But they might give you a good deal on a replacement frame. Good luck.
Last edited by WizardOfBoz; 09-11-23 at 07:17 AM.