Explanation
A custom action on Account can be used to create additional accounts from a parent account record. A custom action can have predefined values for certain fields based on the parent record. A custom global action can be used to create records that do not have any relationship with other objects. A custom link on Account can be used to execute JavaScript or link to an external URL.

Making the Opportunity type a required field on the initial Opportunity page layout and using automation to fill in the type field to a record type is a way to limit the number of fields users are required to fill out when creating a new Opportunity. This way, the app builder can assign different page layouts for different record types based on the Opportunity type. The other options are not feasible or effective

The two reasons that could be preventing this relationship conversion are that there are already two master-detail relationships on the Invoice Line Item and that Invoice Line Item records exist without having the Invoice lookup field populated. A custom object can have up to two master-detail relationships, so adding another one would exceed the limit. Also, a master-detail relationship requires that the detail records have a value in the master lookup field, so any records with a blank Invoice lookup field would prevent the conversion.