I don’t own an X68000 but we’ve had quite a few happy X68000 and OSSC users, hopefully someone more experienced can chime in here too.
1) Given the range of screen modes on offer here, I’d say SCART was a better option, as you may want the LPF active for 15 and 24 khz modes.
2) See above, no special settings are required for basic functionality.
3) OSSC converts all modes to 31khz unless set to pass through, but typically you’d only want to pass through 480i on a modern display.
4) X68000 is tri-sync on its output depending on the software in use. I’m sure the XM29 supports both 15khz and 31khz, but I think only the Plus supports 24khz so you’d have to be mindful of that.