Our baby is just over 7 months. She can only roll back to front to back to her left and not yet to her right but she has just started crawling. After being placed on her back to sleep she is rolling onto her tummy multiple times overnight, sometimes turning her head to the side and sometimes not (which is scary). We are rolling her back onto her back but sometimes this can be a dozen times overnight and often wakes her. Now that she is crawling and has the strength to lift her head (even though not rolling both ways) are we able to let her sleep on her tummy or do we continue to roll her onto her back until she can consistently roll both ways? Thank you

Most babies will be rolling over by 4- 5 months…...some earlier, some later.                           
This is normal as babies grow & become stronger.
Many babies then prefer to sleep on their tummy.
This is not something that needs to be stopped. It is important to make his new environment as safe as possible.

Ensure that the cot mattress is firm & flat & there is nothing soft or loose in her cot, especially as she is face planting at times.
You can try to turn her face if you see her with face down, but often, like rolling to tummy, babies will just go back to their position of comfort.
As you have found, there is not much to be achieved by trying to roll her back from her tummy.
Always continue to place baby on back to sleep.

She is obviously strong in her upper body, if she is crawling - this is important for rolling safely.

Red Nose has the following general information now that she is rolling.

Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lambswool, doonas, pillows, cot bumpers or soft toys)

As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort.
By this stage it is not necessary to wake during the night to turn baby over to the back position

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over