Password reset inconsistencies between O365 and Azure portal. Identifying federated domains.

Hi,
I have some questions around B2B guest accounts.

1. Should we able to reset guest passwords in our Azure AD? We have an option to do so within O365, but AAD says this is not possible. Why is this?
2. Why do guest accounts appear as _hotmail.com#EXT#@contoso.onmicrosoft.com within O365, but @hotmail.com within Azure AD?
3. How can we identify if a partner domain is federated with O365 already?
4. Some of our partners have not completed O365 migration, so they're in a semi migrated state with some users in AAD and others still on premise. This causes issues with B2B. What's the best way to deal with this?
Thanks