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There is a good post here in seomoz about exactly this issue with DMOZ - in recent times due to the sheer size of DMOZ itself, it really doesn't get crawled that well in the more niche or local sections.

There are two great posts in SEOmoz itself that I found very useful;

Competitive backlink analysis

and

Using Google CSE

Both use the tools within SEOmoz itself and will help you mine for link candidates more widely, and then analyse and target the links you want to gain in depth.

With respect to which pages to link to, using the above tools will enable you to link with a wider variety of anchor text to specific pages - keeping the relevance as high as possible. Personally I would be inclined to balance between category and product - if you can get a targeted link for a niche phrase to a specific product page - do it, if the domain you are gaining the link from is in your field, but not product specific, go for the category link.

I have seen/used/tried several tools over the years, and as many have said, the vast majority have not made life that much easier or have been full of spam.

I have used Adgooroo "link insight" with success (no, I am not an affiliate) but it is a bit pricey IMO. Once you have it running the quality of links it will find are impressive (it only pulls back the top 10% of links it finds), but it still requires the leg work to gain the links themselves.

From a logical point of view (or thinking out loud as it were) if you are consistently using the same B page, that in itself will be where the problem is likely to be.

For example - if you canonical the B page, and continue to run the B page anyway, I would think you would end up creating a loop, or at minimum a strange redirect because the script in you A page for the split test to function will/could point it back at page B

If you 301, I would think the same could apply, and still not solve the issue.

If you ran the tests using the multivariant test format instead, I would think this is less likely to cause issues because you are then just switching segments of content on your landing page, rather than urls, therefore eliminating the problem altogether.

Remembering to keep an open mind to new ideas and concepts, testing them, and ensuring that I keep a mental note of both "do more of that" when they work and "not even if hell freezes over" when they don't (except when google changes something).

Submission to directories in general used to be seen as the big way to promote a site, these days however (as has been said by many) generating back links needs to be a bit more thought out than just blasting out submissions. I have used IBP and WebCEO in the past with success, but actual traffic and link juice gained is very nominal.

Personally, once submitted to the major SE's - Google/Bing/Yahoo etc, I would spend the time/money on specific link mining and building instead.