Presuming you did not create the original sandbox, and you logged in to that sandbox at some point, and you never updated your email address in that sandbox, this is definitely something that could happen. Make sure you log in to the original sandbox, update your email address, then refresh the new sandbox you created. Alternatively, you can have another ...
I'm not aware of any such feature ever existing (the ability to spin up or refresh a Sandbox from another one), so I can't speak to that exactly, but you can still clone a sandbox today under certain circumstances, but only from Production
Hope this helps.
The sfdx structure is meant to be used in a repo. The folder structure is not uploaded to Salesforce during sfdx deployments, and thus cannot later be retrieved. You can deploy the same code to multiple orgs using sfdx, and can even push/pull from multiple scratch orgs, as the metadata is saved per org in the sfdx configuration files.
Yes, definitely possible, you will need to enable multi-org for that.
You will also need two Marketing Cloud API Users, one Service Cloud API user and on Sandbox API user.
Here is the outline of the connection:
PROD BU ---- MC PROD API User ---- Service Cloud API User
Sandbox BU ---- MC Sandbox API User ---- Sandbox API User
Read more about Multi-Org ...
It is possible, but it requires configuring the entire Marketing Cloud instance to a Multi-Org instance, which is not reversible.
To enable this, you'll need to contact SFMC Support. Check the FAQ to understand all the implications and limitations before you request it.