I would do any development that a the customer requires on the customer branch and merge it into default. This will work for hotfixes and it is simpler than cherry-picking your development changesets from your default branch. It's easier to merge forwards rather than backwards. It also gets rid of Type-B file problem because there are no Type-B files in the customer branch.
Type C files I would merge into default and then delete on the default branch. Any further modifications to these files should generate a warning that file modified on branch was deleted on the other branch.
