Hi there, this time can be so tough with developmental change and also nap dropping.
I can understand why you wouldn't be keen on a lock on her door, I wouldn't go with this either. I do know others here have had success using a stair-gate on the LO's bedroom door. This way you are providing a physical barrier to prevent her coming out when you are enforcing sleep-time but she can see and hear you in case she needs reassurance, and you can hear her to make sure she is ok. Might be worth a try? We haven't done this personally, although we do have a gate at the top of our stairs (which are immediately opposite DDs bedroom). When we are struggling to get her into bed at night I do find it helps us to have the gate closed and us downstairs. She can, if she wishes, walk around between her room and the bathroom but not really any further, and this small bit of freedom does seem to help her feel relaxed and eventually she does settle and sleep in her bed.
I agree that it might be worth trying a CN (perhaps a car nap) to see if that helps her catch up a bit and then you get a better night. Unfortunately when Los drop the nap cold turkey you are going to experience OT, so any little catch up you can get in should help. Then perhaps try and keep the day no longer than 11.5 hours. I know some here say no more than 11 hours to start with when first dropping the nap.
Have you tried EBT at all? If you feel she is really OT on any particular day, perhaps try a much earlier BT - say 5/5.30? It sounds scary, but it can really help My DD never tacked on at all until this transition, but she managed it and I was amazed.
Have you tried starting the idea of quiet time during her old nap time? The stair gate could help this, she has to stay in her room and play with quiet toys in there, or just lie down or look at books, but there is a fixed period of time and you can in theory still get to have some down-time. It might help her feel a little rested too. This never worked for my DD, but it does work for many.
Do you feel like any of this could help?