does JMP accept payments in Monero (XMR)
Yes. It's automated for top ups but not for signup yet, but you can contact support https://jmp.chat/faq#support to get an XMR address for signup purposes.
would I still be able to use the actual phone to dial 911 if necessary?
Carriers in USA are required by law to complete 911 calls even if you are not a customer and have no SIM card at all. So yes.
I saw something about an ESIM adapter that’s a regular SIM card that can have multiple ESIM profiles on it. Why can’t I just buy this directly?
You can: https://jmp.chat/esim-adapter
If you try the sign up process you will see what payment methods are supported. Namely: credit card, bitcoin, payment by mail, or (in Canada) interac e-transfer.
If you contact support you can get an address for ETH or XMR as well for signup, these are not fully automated yet.