Single or multiple use cases for alternative actions in the same flow?

There are two aspects: the documentation of the use case and the UML Use Case diagram. The UML only specifies the Use Case diagram. My preference is to focus on textual and tabular representations of the use cases and minimize my use of Use Case diagrams, which is something also recommended by Martin Fowler.

My initial thought would be to create a single use case, and "Manage Offer" is a good name for it. The "happy path" of accepting an offer would likely be the "Accept an Offer" path. Rejecting offers and making counteroffers would be variation scenarios or extension paths.

Depending on the complexity of the flows, I would consider splitting out the extension flows. Extension flows, at least in Cockburn's templates, don't necessarily require the steps to be enumerated within the use case. The extension could refer to another use case table by name.

If I had to create a Use Case diagram, the use cases that I'd include depend on the complexity. A comprehensive Use Case diagram would consist of not only the primary use cases (such as "Manage Offer"), but also the sub-cases using extend and include relationships as necessary. But this could make the diagram more difficult to read. If including the sub-cases made the diagram too noisy or cluttered, I'd drop them in favor of the top-level use cases only. This allows the diagram to, essentially, serve as a map to the tabular structures that document the details of the use cases.