The answer is to combine referential attributes in the subtypes.
In the past, BridgePoint has had the ability to combine referential attributes that refer to the same base attribute as a way to enforce constraints in the data model. Recently, the rules of when you can combine referential attributes have loosened and you can now combine any two referential attributes of the same type (they don’t need to refer to the same base). See this issue for more on that: https://support.onefact.net/issues/9446, https://github.com/xtuml/bridgepoint/blob/master/doc-bridgepoint/notes/9446_merge_attrs_int.md
Attributes can be combined by selecting one of the attributes, right click -> Combine With… and selecting the attribute that you want to collapse into the attribute that was selected.
To answer the second part of your question, I don’t know of any reason historically why adding a prefix is not part of the formalization wizard for sub/super associations. But of course it is possible to modify the prefix (or use the prefix from the super class) through the properties view.
Hope this helps!