My understanding is the Merlin sleep suit is for transition out of the swaddle (around 4 months old for my baby). In my opinion, she is definitely old enough and strong enough to be in a regular sleep sack so she can move around to get comfortable. It’s definitely ok to let baby roll to her tummy to sleep. Pediatricians recommend still placing baby on their back to sleep but no need to flip them back if they roll to their tummy.
I believe the merlin sleepsuit is only safe up until baby can roll in it. My son started to roll in it around 4 months and we switched him to their sleepsack and he was ready, he still sleeps through the night.
I had mine in the sleep suit till a few weeks ago and she would roll around like crazy. If you’re already cosleeping I’d suggest to just have baby sleep without it. It’ll take a few nights for her to get used to not feeling “restricted” but the process is inevitable and you’re going through rough nights as it is 🤷🏻♀️ it’ll get better after she feels more comfortable.
Might be ready for sleeping in their tummy and a light sleep sack