Sounds like a big and risky endeavor, although there could be potential benefits.

Why are you considering merging the sites if they are already ranking well for their keywords?  Are they not ranking high enough?  If it ain't broke...

A better strategy might be to just build more links and add more content to the sites you have.  Another area you could focus your efforts on is conversion rate optimization.  You're already getting all this traffic for desirable keywords, so you can boost your client's ROI by figuring out how to turn more of that traffic into leads.

Hey Samuel,

Multiple links from one site can help with rankings:

1. If you are targeting different keywords (anchor texts) with each post, you can improve your rankings for different keywords with multiple posts on the same blog.
2. As posts get older and are pushed off the homepage into the archives, their link value diminishes. Getting a fresh link up on the homepage can thus boost your rankings even if you wrote for that blog previously in the past.

So getting multiple guest posts on the same blog can still be beneficial. Also getting multiple posts on the same blog may be more beneficial than getting posts from different blogs if there is a disparity in domain authority. Getting multiple links from the New York Times is better than having links from 10 random blogs.

The cache date that you see in the search results does not necessarily correspond with the last time your site was indexed.  Your site may have been re-indexed with your new content today, but the cache might still show your page from a week ago.

The best way to improve crawling and indexing is to improve your PageRank.  The higher your PageRank, the more resources Google will be willing to spend on your site.

PageRank and mozRank are good metrics to look at when evaluating a blog.

I'm not sure how wide a net you want to cast with your contest, but I wouldn't be too quick to ignore links from even relatively low PR sites. As long as the site is in the same niche, getting links from a bunch of relevant sites (especially with good anchor text) can do more for your pages' rankings than just increasing your domain authority.