you can do relevant and quality back links through forum and commenting on blogs. Relevancy sounds good on back links.
Yes, Google may remove back links from its index if it finds it irrelevant anytime. Build reputation of your website with QUALITY back links and FRESH n REGULAR content.
Right. I've also contacted others in my niche and asked if they wanted to trade links .. These are GREAT permanent links back to your site usually displayed prominanently by the other site owners... Check the sites you linked to, are they still up or are you loosing backlinks because the sites are down??
I had in the past similar problems with a website. In webmaster tools the links are vanished about with 40 procents. About 2 weeks webmaster tools indicate correct links - old links + old links (lost) + new links. Don't worry. Google makes changes these days...
Are you using this query to look up your backlinks? link:www.example.com If so, it won't be accurate. To get a more accurate reading, you should use the Google Webmaster Tool.
I wouldn't recommend buying links. Most probably would be irrelevant. Look for ways to build Quality links, such as forums, article submissions, bookmarking sites, etc.
My theory on this is that if the links in GWT is showing less than what you actually have (as seen in yahoo backlinks) then the links have are not being counted by google are deemed worthless. It might be because the webpages on which your links are are no longer "important" or the links have been devalued. Either way, you need to keep on building links and by that i mean quality links.
Such thing is normal happening while the owner deletde your post or deleted the thread. Right, just keeping on building more backlinks.