Its a simple fix to make it play full speed and full screen, although you may be playing on a LCD and it probably doesn't matter.
As above, both work great - I have a Darksoft version, very nice it is too.
Have you checked the back of your TV, most uk televisions of that era had a scart socket, if you want to run arcade boards you need one.
As above that scart to phono adaptor you have will not work.
Alternatively it’s a simple mod to swap the speaker connections around, your cab is stereo the 1C is mono. On my multi slot cab I fitted a switch, to switch between MVS and Jamma.
Like this. https://www.ukvac.com/forum/threads/jamma-mvs-mono-stereo-switch.59070/
I managed to fit a Hantarex 21 inch monitor in a UK wooden Neo Geo cab, stock monitor was 19inch… I think. But I defiantly went up a size and all good.
Good luck, it’s going to be a tight fit for sure, those big monitors are heavy.