[Chris Alaimo] “The video signal comes back out of that monitor, into the line multiplier, and then into my capture device.”
That video signal is just passing ‘NTSC’ video and audio, as evidenced by the CRT TV.
RGB is color, and has nothing to do with frame rate or “resolution” of which were lines in TV, including old console games. RGB simply carries color differently than, say, composite or s-video. SCART was a connector that allowed all of this to be bundled in to one cable, and didn’t define any resolution, only signal (audio/video/etc).
15KHz is only SD. If you are saying 240p, that’s just 480i doubled, meaning it’s writing each line twice (at 30fps). It’s still interlaced, or more accurately Progressive segmented frames (Psf), and you are getting nothing more than NTSC/PAL video (at best) out of that rig.
The scalar, or whatever that is, is quadrupling the lines is obviously amending and converting the signal from analog to digital, and doesn’t stick to “tv” standards because it doesn’t have to since it’s used for computers which are, for the most part, resolution and frame rate agnostic. It might be sending “~60fps” to the computer, but it’s certainly not receiving a 60p analog video input.
You would probably be able to rewrite the frame rate in those video files and be just fine (Cinema Tools used to do it, but that is long gone). If you could tell your capture software to write the header at 59.94 (or 60.0), those files would also work better in FCPX. Perhaps there’s a way to tell the software to lock the frame rate to a more standard frame rate? Or try capturing in a more standard “tv friendly” format?