At work, I have been handed some old.cp files created circa 2009. They were used to build a SCORM package in Flash for use in a course on Moodle. Flash is of course at its end-of-life so we now have to rebuild that package in HTML5 instead.
What is the latest version of Captivate that will give me the ability to open .cp files so I can view them and resave as .cptx?
I do not know in which version the .cp files were originally created as they were created by a contractor.
Probably none... You should need version 5 because the switch to cptx, where the old version of AS was no longer supported happened with that version. It means your projects were created with an even older version. CP5 is over 10 years old.
Meanwhile so much has changed, that my recommendation would be to rebuild from scratch because that will probably save you more time than trying to find several older versions of Captivate and make multiple updates. For each update you would have to find the non-supported features and replace them.
I even doubt it is possible to open the Library as an external Library in a new version. That is possible from CP5 on.
If those are very simple courses, not much interactivity, you could capture the course with the Video Demo tool. The output will be MP4 which you can convert to an interactive video to add some interactions.