Yes, you can fire .40. Note that you will be headspacing on the extractor rather than on the mouth of the casing. So, your extractor must be functioning properrly and repeated firing may lead to extractor wear or failure.
I run a bit of .40 through all my 10s (both). This confirms viability of the firearm as a platform to fire .40 if it ever gets that bad. 40 has run fine in all 4 of the 10s I’ve tried.
It is a good practice to clean the chamber after firing 40. The chamber rim 10mm headspaces on has been exposed to blast from the 40. Clean it to ensure 10 finds battery properly.
On the magazine front - they can run. I’m careful to load them flush with the back wall of the mag. I too can run 10mm from 45ACP Redballs but it just doesn’t feed smooth enough for warm and fuzzies yet.