Hi JWL,
Here are my two cents:
1. It is possible to add an condition to have the workflow rejected when there are no settlements. This will need a small customization. The customization can also be done outside the workflow, but during maintaining the payment journal.
2. I assume this is before the workflow, so should not be an issue. It can be useful to do this on purpose. e.g. pay half of the invoice amount.
3. The journal is a worksheet. If you setup workflow, a user should check all details prior to generating a payment file and do the posting.
Indeed you can add all kinds of payments apart from invoices. This is for some flexibility. Your company must be in control to check the journal prior to generating the files. A workflow can help you. When a journal is approved, no more changes can be made.