Heard a new one today

Pretty common. They should've cleaned up the beads before installing new tires.

SO......if they already knew that from when they installed the tires......WHY THE HELL DID THEY NOT TELL YOU THAT BEFORE ????

Anyway, they should be able to show you two things:
First the bubbles from where the leak IS.
And then the bead of the rim with the tire off so you can see the supposed "corrosion".

While their story is possible, it usually only happens in areas where they put salt or other chemicals on the roads in the winter.....or if you drive in the ocean on a regular basis.

Yes, it's possible. Does it appear that your vehicle may have enough corrosion for this to occur?

Quite possible. The 100k is irrelevant. Only depends on the condition of the wheel. Bead sealer should take care of it.

Have them apply a dab of bearing grease all around the bead. Drive them and see what happens.