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Some fluctuations (between PR 3 and PR 2 in your case) do not mean anything bad. That's just natural. As for the traffic, you didn't mention how huge was the drop in traffic and position? Chances are, that's another normal fluctuation

I have been guest posting for a little while now and I can tell you from experience good sites (authoritative ones as listed above) are not easy to guest blog on. You will need to give the author of the website something valuable to his readers. I noticed you are focusing on little details on what Google counts and what they dont. IMO forget all that, give the users something valuable so that they will like your site, mention it, and bookmark it. If users like your site so will Google!