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yes we can increase traffic to our site by joining many forum and contributing value post

Forum signatures bring backlinks and more traffic to my site. In forums content is updated frequent so links seem to be natural

It help to get backlinks and also direct traffic ( this will be depend on forums be relevant with your site or not)
I think so

Blog comments often contain nofollow links, and forum have both nofollow and dofollow links

First you should focus on content more and more, and post links in high PR site

I think you'd better use Goolge webmaster tools, in which you can find the top searching of your site, number of click ( detail for each keywords)

In order to be at the top of SERPs, you should analyse your competitor who is the number one on SERPs, try to have more quality backlinks than them, focus on content more and more, you will win

onpage SEO is necessary for SEO however Goolge change their algorithm, the problem is considering human readers as the centre of all action of SEO

Yes, I've baned several time when I did'nt understand rules of some forums and now I learn from this and try to avoid this :)

I think before getting links from forums you must contribute some valuable post. The question how many post for each forum depend on each forum, but I think we join forum to discuss and get useful...

Use Google webmaster tools for your own.
Other sites you can use: SEO spyglass, backlinkwatch, majesticseo