Hello,
if yo uwant to test that your HA installation is ok, please follow the guide here :
Add devices without a phone
I tried this feature recently and it work very well !
About the thread credentials, I managed to borrow an android from a friend , installing companion app and all, and it worked at first try !
I believe the reset of thread credentials is very very glitchy and once you poison your phone it’s very hard to change them…