Microsoft Graph API is the API that gives you access to Microsoft 365 (Formerly Office 365). From your description:
- Creating office 365 accounts can be done through Graph API if the subscription comes with an Azure AD subscription.
- Deleting accounts would also be possible through Graph because you can remove licenses and even delete accounts through Graph API.
You can state what Microsoft 365 (Formerly Office 365) plan you are one to get a better answer.
The other actions like set signatures will depend on the M365 service you want to set that on. Graph is a gateway so features have to be supported by the target M365 service.