disconnecting an iCloud account from a device

Can you not go back to Settings?

Note that I have not tested this myself (so I could be wrong). as I have always done the steps in order as described in What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

If you can't do it on the phone, it may be removed. You could verify by using another device trying the "Unlock on the web" section of How to remove Activation Lock - Apple Support

No. You also need to perform Step 3 in the following: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support