Merge/move 2 AD's and 1 Office 365 tenant to new AD

Hi,

This is based purely on what you have outlined above and it's a personal opinion - others will have their own.

1.) Cut AAD Connect on Location two so the AD and 365 tenant is separate.
2.) Set all 365 users UPN to @company.com, mail addresses remain the same
3.) Build new Forest corp.company.com
4.) Consolidate the AD's into the new forest
5.) In AD all users to have a UPN Suffix of @company.com, or UPN of @company.com
6.) Install Member Server/Azure AD Connect
7.) Ensure the AD objects have the right mail address
8.) Soft match the users from new Forest to 365

That's a general overview. It's probably best to take a step back and disentangle what's there first even if it means having cloud users and separate credentials for a period of time.

Hope that helps

Best, Chris