is there a difference if I downzap something with 100 sats at once, or if I downzap it twice with 50?

No difference. It works just like upzaps. We consider the entire zap amount as a whole and even end up using some basic algebra to do this.

that is why I started to wonder whether the unpaywall comment does not count towards the the upvote of those specific posts

Outlawing does not consider comments. Only upzaps and downzaps.