I'd agree with your conclusion that your rugosa seedling came from glauca pollen.
I got identical looking seedlings when I did that cross intentionally. You can see some
old picture on HelpMeFind:
I tried growing out an F2 population but many of them were weak and sickly.
A backcross to rugosa was much better but I didn't grow enough to see if any might rebloom.
I did have at least one that retained the beautiful blue foliage but was more rugosa-like in
appearance. Like glauca, these seedlings were very susceptible to RRD. I have a small
piece of one of the F1 and also one of the backcross seedlings still alive, but all the others
I like Philip's idea of using foliolosa and/or fedtschenkoana with it. That would be fun to see!
Good luck with wherever you take it! Tom