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Are you saying to treat "Service > Blue Widgets" as page 1 and "Guide > Blue Widgets" as page 2? I think Google would likely ignore rel=prev/next in that context, but it's hard to say. You might be better off using rel=canonical. I'm not sure that both version have great search value, honestly. I've seen rel=prev/next used on a series that wasn't strictly paginated, but where there was a clear order. Unfortunately, it can be really tough to measure the effectiveness of rel=prev/next, even on fairly large sites.

That's very odd - I'm afraid I haven't seen anything like that. A couple of questions: (1) Does your analytics data match the GWT data? If so, can you segment it at all? (2) Is there any reason Thursday might be meaningful? For example, do you publish new content on Thursday or resubmit your sitemap (sometimes, even little things can have an impact)? (3) Do your logs show anything suspicious, such as bot activity on Thursdays? Do your logs match this data at all? If all three are "no", I'd venture to say it's a GWT glitch and not to lose sleep over it. If it is a Penguin update test (and I'm not seeing evidence of a weekly pattern), you should see the impression changes in your analytics and logs. They shouldn't just be an artifact in GWT.