If the reports are not embedded in an application, then it should be fairly easy locate them in a folder and build a link to them in the application. in our application, we had the file name/path along with a display name in a table. Not too familar with Betrieve, but I would assume something like that is doable. If there are already existing Crystal reports in the POS, licensing shouldn't be an issue...but I will be honest, I don't know anything about licensing and its technicalities.
In general, if the user doesn't have Crystal, then they cannot change the fields on the report. Even if they did, I would recommend that they don't unless they know the POS database design as they might get misleading results. If they purchase Crystal, the licensing issue is gone as they own a license.
As for learning Crystal, I like Brian's book the best, and if you are going to be doing a lot calls from an app, the programming books might be an excellent choice. My company doesn't make too many calls into Crystal from the app, so I haven't had a need to get his programming for Crystal books, but I would without a second thought if I had the need.
That he also has this site helps all us is another plus.
Reporting services is very similar, but I don't know if it Betrieve compatible, I have only used it with SQL server (where it is a free package)
HTH