http://www.apelectric.com/6241-Generac-Guardian-Series-14kW-p/6241.htm

This is the code I used:

I do use it. Whether or not it is a good or bad thing I can't really tell you. In my opinion, it is the best practice because it gives both the searcher and the engine the optimal information and provides the best experience for both. I use the discontinued tag on the pages that I don't 404.

I actually prefer the blog be on a subdomain if there are going to be several back links on it. It is a way of protecting your root domain from a link that might have seemed good, but has a spammy link neighborhood or turns out to be negative. It is my personal preference. The downfall is that the traffic isn't going to your site, it is going to your subdomain which is why the best practice is to use /blog.

I would double check with your site developer/IT person to find out if it is really necessary to use the subdomain. Is this by change a WP plugin? It is my understanding that there are several eCommerce platforms with no integrated blog and the WP plugin is only compatible on a subdomain.

Really it is only the name, address and phone number that needs to be consistent. The description should be tailored to the potential audience of that directory.

https://www.youtube.com/watch?v=mQZY7EmjbMA - Here is a video on how duplicate content is handled by Google.

Here are my thoughts, it would be better for you because the searcher who types in the specific make and model in your inventory will be able to find it quickly, as opposed to having to click through a site. Theoretically, that is the benefit. It could also be that pages don't rank because all of the content is viewed as duplicate.

I would make the decision based on what is best for the searcher, which would be to have an organic SERP with exactly what they are looking for.

If a URL is labeled a 404 it will not be crawled again unless there is a reason to, you mark it as fixed, or you edit the link in some form or fashion. Mark it as fixed and see if the error comes back. There is no harm in doing this.

Can you get to the page on your mobile device just by clicking through your site? If you can, that is good, it will eventually encourage a mobile bot to crawl it. If you can fetch and render as google, then I would just give it some time. I am not sure if there is a string of code you can add to the head of that page telling the robots that it is a mobile only page. I don't know how that works.

I would just mark it as fixed right now and see what happens over the next couple of days.

True, but if content in tabs aren't crawled neither are links in tabs. You would want those links crawled. I believe they will be crawled, but I also agree with you that the content should stay on one page.

Take a look at this discussion:

http://moz.com/community/q/content-in-tabs

Did you look at your Mobile 404 errors? Google uses a different bot for mobile sites and anything related to that mobile page. Chances are, if it isn't reflecting a 404 in the Mobile errors in GWT, it is being indexed properly.

Check it out from you phone. Google the exact keyword and your company name. See if you can get to the page and if it is in fact the correct page.