Where is a Romney or Ryan going to get delegates from?
At this date they could not even get on the ballot
The state GOPe’s can revised and add candidates for primaries and caucuses at will. The Republication Party is a private entity so they can make rules up as they go.
They have a dilemma right now whether to put Romney in the race and have him try to win enough delegates to stop Trump or whether to try and get him on enough ballots as an independent candidate to give the election to Hillary. Only about eleven states are available where that could occur.