My 6.5month old has her first cold and will still roll to sleep on her tummy rather than her back. Will she be safe if she rolls there herself with her semi congested nose as she won’t stay to sleep on her back?

Has she now recovered from her cold?
Red Nose has some brief information for bedding when baby has a cold .
https://rednose.org.au/article/bedding-for-babies-who-have-a-cold

Most babies are rolling by 4 - 5 mths & most prefer to sleep on their tummy or side.
Still always place you baby on their back.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over