Responses

You reading this spin-off won't affect your reading of the main series that much. Aside from the pd, no other character appears there. It's about a different couple and the pd might be a "2nd ml" but I think his presence wasn't really equal to the 1st ml.
Also the spin-off is mix of past and present, so the main series is actually in between. So the only thing you've basically spoiled to yourself about the main series is who's the end game.
I personally thing the two webtoons give different vibe. This one is more mature and serious. While the other is a bit cutesy and funny.
Goddammit I didn't know this was a spin-off, did I saw spoilers from the main series? Is it worth reading the main series even though I read the spin-off first?