The answers I got from Happycube:
1. I hacked up a video capture card to work as a direct ADC, then capture the raw RF signal from a test point on the Laserdisc player. This bypasses the signal processing/video output electronics, while the player itself works normally.
The raw signal is then demodulated into (analog) audio and video, then framing and comb filtering/color decoding are done in software. http://imgur.com/a/7VNff shows each stage of the process.
2-3. I think the X0 would win in either case, at least until the analog->digital conversion is upgraded. This is the best direct comparison I could come up with between the X0 and ld-decode (using a CLD-V2800, a rather low end player!): http://screenshotcomparison.com/comparison/128845
If my tinkering with an RF amp circuit is successful, the gap might close a little bit. Eventually I may cook up a circuit using the LPCLink2 (with the same LPC4370 chip found in the AirSpy), which I suspect will give better results.
4. Not much at all, I need good quality VHS RF samples (and/or the time to hack up a VHS deck, I've got a couple of Panasonics to butcher sooner or later). Also note that dealing with Macrovision will not be a priority.