Hi there,
It certainly seems like your management has put you in a tough position. It sounds like you may have gone through a process of Azure AD registering your work device, which is generally not a recommended path for domain-joined devices.
The user accounts for things such as Teams, are they sourced from a different Active Directory environment, or are they sourced from the Active Directory you have control over?
Using either Azure AD Hybrid Join, https://learn.microsoft.com/en-us/azure/active-directory/devices/howto-hybrid-azure-ad-join, or Azure AD Seamless SSO, https://learn.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-sso, are the two mechanisms for organizations to provide corporate-wide SSO to Azure AD/O365 resources.
To set either of those up, though, you need the users to be sourced from your Active Directory environment, and you need access to the Azure AD Connect server with Global Administrator credentials to configure Azure AD Connect. If the users are in a different Active Directory domain, it further complicates things. If the users are "cloud-native", in that they are not sourced from Active Directory, the recommendation would be to go through a process of connecting them to your Active Directory users, unless there is a bigger plan to perform a domain migration with your merger & acquisition scenario.
It may feel complicated, especially if you do not have a lot of experience with Azure AD, but sorting these things out using a recommended method is going to provide the least headache long-term. If you want to elaborate further on what the environments look like from an Active Directory perspective, please feel free to do so and we can talk this out further here.