It is very unlikely that vaginal estrogen would cause cancer to develop between now and then.
You say you have started progesterone - I would actually use a progestin in this situation.
If you are going to be waiting months a mirena IUS could be considered.
This is actually being used as a treatment for endometrial hyperplasia.
I personally wouldn't have any concerns about using vaginal estrogen with an IUS in this situation.
Your cell changes have been picked up now and there is no danger of you falling through the net.
Alternatively there is oral ospemiphene, a SERM that can be used to treat VA. This can be harder to access and I don't know if it is available on the NHS but worth looking into if you are interested.