Yes, I have been using SimpleX for quite a while now.
Oh no, I didn’t have any problem registering with #nostrcheck. I went to the nostrcheck.me website and registered with a username, password and my public key. They actually offer five domains and I chose the nostrverse one and made a verified address. I can't understand what your issue might be. But you can always ask question using the #nostrcheck hashtag or directly ask quentin (npub138s…x624) about it, maybe!
Good luck, Mike! 💜