The salvage yard has 3 other Focuses and all them are crusty. I suspect a lot of them end up in yards up here because people don’t bother washing them frequently in the winter or undercoating them. This Gen Focus tends to rust real fast.Can you go back to the salvage yard where you got the used fender and just pull the special tapered bolt? I'd imagine they would sell it for a buck or two instead of $28.
As long as the LCA is loaded to the same angle relative to sitting on the ground, tightening the bolts will be fine. You will need a wheel alignment afterwards, so that should resolve any minor discrepancies.
I just jacked the LCA up to a similar angle as it would if were it on the ground and got the bolts torqued to 85% of their factory spec because I have everything covered in anti seize.