Anything related to your keywords/niche will help you.Blog commenting and blog posts help in SEO significantly but you need to make sure that its all related to your content and niche.
well, in the early start: yes! but after you start to appear in SERP you should be more careful in posting comments that are not related to your niche as they may drop you from search results.
I feel that relevancy is not as relevant as everyone says. At some point, you exhaust relevant opportunities, or at least highly relevant opportunities. The quality of the links (type of BL, its position on the webpage, age of domain, pr of page, IP address, etc..) in my opinion, are what matter most. I do try to get relevant links where possible, but to me, the reality is that relevancy is not long, long term sustainable. This of course may vary depending on the nice.
Backlinks which are surround by related content can work wonders for your site, always remember that Quality beats Quantity any day of the week. I would just concentrate on content relevant backlinks