You'd want to be holding Option until the blue startup screen to disable Basic.
Pre-XL games, it's a must do in many cases, later on a lot of the programmers were just lazy and printed it in the manual to do so rather than spending the entire 8 bytes or so of program code to do the same.
It comes down to a case of dependancy - at each cycle the program can change a single hardware register that can affect the behaviour of most of the machine.
And a case of accuracy - no point having emulated Antic+GTIA render an entire scanline worth of pixels if the CPU goes and changes stuff midway - it effectively makes most of that work pointless because of the changed state.
There's stuff that could get farmed off to other cores - I think Avery explained some of it elsewhere - but the overall benefit of farming that work out isn't particularly great.
In theory you could have 4 threads e.g. doing CPU, Pokey, Antic, GTIA - but the thing is that each thread would have to wait on the others, never getting a cycle beyond what's going on with the others. That means some inter-thread communication required, not to mention the fact that each thread needs to be active which in a multitasking Windows environment is no way guaranteed. Alternatively of course, a thread that's completed it's task can go into a wait and allow the other thread/s to complete their little bit of work unit on the same core (assuming the Win scheduler executes it).
At the end of the day, a bunch of extra complication in the overall system, possibly for not too much gain.
Sync is composite sync which is sourced from the motherboard. I had the same problem first up, had forgotten to put in that connection. If the monitor has some selection ability, it'd need to be seperate composite sync, VBXE doesn't use Sync on Green, or seperate H/V Sync.
In some cases where only seperate syncs are provided, the usual is to tie the CSync output to HSync on the monitor.
If you're getting sync problems with normal video as well then that'd indicate a problem somewhere else.
Colour adjustment Pot will make absolutely no difference to RGB, VBXE video is independant except for the shared CSync.
Sound - I guess you source that from the normal monitor port? In any case, I'd be getting the video output right before worrying about sound.