What i was thinking about is if its possible for a bitcoin address to be reversed to the public key? And then that public key reversed to see which xpub created it? I remember reading something about that.
This is not possible.
I know it's not possible to know which master private key created the xpub but I don't know about the other things. Because if you can with an address do some algorithm to see which xpub it comes from, then you can find out all the other addresses that the xpub has created. Is any of this possible?
Maybe the address has been used to make transaction. If an address is used to make transaction, the public key will be known and that is just normal.
Can I without worry share my seed phrase publicly online if I only use a hidden wallet which uses a strong passphrase?
Not advisable.
A short/weak passphrase will be easy to brute force. A very strong passphrase will be hard to brute force but not advisable.