Access SharePoint document library with a Microsoft Account (not Azure AD Account)

I am creating a Web application with ASP.Net MVC and have registered it in my Azure tenant for Azure authentication.

The application is using infividual accounts and stores accounts in a SQL database, because I want to invite outside users without having to create azure accounts in my tenant.

Users need to upload files to a SharePoint document library.

For users inside my tenant I request the authentication token and all can upload files successfully.

For external users I have registered an App at I have shared the document Library to some external Microsoft Accounts to make it available for them through my application.

When external users hit my application, they have the option to select Office 365 logon or Windows Account logon.

When they log on with a Microsoft account in the usual Windows Live account logon, an account is created in my database. With this account the users can't access SharePoint, because I can't obtain a token, even though I shared the document library for their e-mail address.

Although when they logon with their Microsoft account through the azure logon (which redirects them to the Windows Live account logon) a second account is created with the prefix with the same email address. They have to access the SharePoint document library through my SharePoint though before the login, in order an Azure Account is created automatically in my Azure tenant. With this account they are able to access SharePoint.

Is there no unified access available where external users just use their Microsoft account without having to create the Guest account in the Azure Tenant and the need to open SharePoint before the document library can be used in my application?

Are there any other options available to use SharePoint in a web application which make the handling easier for the user and developer?

If anyone has had the same issue before, help would be appreciated