can't fill in password on xbox one console, keypad doesn't come up, head set now doesn't...

Try to
Factory Reset your console. Note: Choose "Keep all games and apps" to not lose any installations.

Or try power cycle console described on the bottom of the page.

There are a couple of options that will allow you to still be able to get that password reset for your gamertag. If you go to account.live.com and click on Can't access your account. Enter the account email and caption and then click next that will take
you to the password reset options. Click on the blue link that says I can't Use any of these options. That will have you provide an alternate email and fill out a questionnaire. The process usually takes between 20 minutes to 2 hours to get a response.

If that doesn't work to reset the password. The next step is to phone or chat into xbox support and they can escalate your account up for a password reset from the account team. They will need the password request number from that form you filled out and
the console id from your xbox. That can be found on the 360 by going to settings, system, console settings, system info. Second number at the top of the screen will be the console id.

I've now got control over the folder, but it still doesn't let me set it as the default install location. When I try to, it comes up with the error message 'You can't set your default install location to that folder. Try another one.'

You didn't read the first sentence of my post. Please read message other than subject heading. Disconnecting worked, but now I can't reconnect. There's a bug where the virtual keyboard doesn't appear. I can't reconnect to wifi if I can't type password.