I do not know how to do that with the JRiver files as the are not in one file like a regular MPEG.
There are multiple files if you are using the JTV recording format. In that case just play the JTV file. But only an installation of MC can do that, and you need the full set of files from the recording available to do that.
If you are recording in TS format, there will be just one file for each recording. Any JTV files that you find are left over Time Shifting files, not scheduled recordings.
PS:I posted the above before I read this new post from you.
I was able to watch one show from about 3 months ago for ten minutes and the picture froze while the sound continued and another did the opposite with the sound freezing and the picture continuing. one of the recordings continued after using the skip function and completely froze up after another few minutes. Older recordings will not play at all. Yhey just display the JRiver icon in the middle of the screen with a message about something went wrong.
I'm sorry but that really sounds like an ongoing hard disk failure that has been occurring for some time. The only alternative is that something on you PC is corrupting the contents fo the drive.
I use and recommend
HD Sentinel (note that the site is either down or very slow right at this moment) as it is very good at predicting hard drive failure. I don't know how you verified the drive yesterday, but I think you need to look at it more carefully.
If you find and download some sample files, make sure you save them to your SSD for testing, to eliminate the hard drive as a potential failure point.