YouTube doesn't always come out and say exactly what their algorithm does but I think it's safe to make some inferences. They have stated that user engagement on a video helps the video ranking. Thumbs up/ thumbs down, comments etc all help. Is YouTube's algorithm smart enough to know that you had a comment that you then deleted? I don't know. My gut feeling is deleting comments will hurt your seo because when the google bots re crawl your video it's got less comments. I making an educated guess here.
Monetizing videos should actually help your SEO if all other factors remain the same (views, watch duration, etc). YouTube makes money from serving ads. It makes sense that they actually give a boost in rankings to videos that are serving their ads. It's the same thing Google does with websites. If a website also advertises on Google back when Google used "page rank" as their system they would boost a websites page rank by 10%. Pretty safe bet that Google likes people to monetize.
Hope that helps.
