It will be close, but I think you may be able to use the larger panniers without heel strike.

I assumed a lot, but here is the rationale:
I wear a size ten or 44 shoe. Your 48 shoes are about a size 13. A size 13 is very close to an inch longer than a size 10.

My chainstay is 415 mm not quite an inch shorter than yours. It might work. There is often a little fore and aft flexibility in pannier mounts. Can you borrow a set of larger rear panniers from your your tour leader to try out?

It sounds like a great opportunity, and it seems like you've thought things through really well.