How can I change device related family sharing settings ?

I did set up family sharing. On one of my devices email accounts are since then greyed out - I can´t change anything about them on that iPad anymore. I searched but did not find a way to change the settings so that I can handle all accounts on this device again.

Do you have a hint for me ?

My second problem is that I try to invite my wife to family sharing, but get the message that I can´t invite my own account. But her account is different from mine and her account is safed nowhere on the device that I try to invite her with. Any ideas ?