Generally I'd avoid hashtags or hashbangs if you have large amounts of content you want indexed behind a hashbang. Use pushState instead whenever it makes sense for the user to actually change the URL.

The general rule is that if you can see the content in your page source (ctrl+u version), it's probably being indexed. That means that client-side AJAX behind hashbangs is generally not indexed, where server-side will generally get indexed.

If for some reason you must use hashbangs, AND you must use client-rendering content, create an HTML snapshot of your page for Google. Generally, though, that's more effort than changing one of the above.

If you do have links sending traffic, those should be the first to update to avoid the slight but unnecessary burden and delay of a redirect. For Google, though, it doesn't really matter. As long as you 301 the whole site correctly to new URLs and change the address in Webmaster Tools, there will be no additional load time.

It's true that you want to avoid big .htaccess files, redirect chains, and inefficient redirects, but that's a rule for live sites. For example, if you were wanting to maintain the domain but move the entire category around, you wouldn't want to add 3000 lines to .htaccess. You should still write a rule rather than going line by line, but it doesn't really matter if it's on an old site that's moving. Search engines will only have to access it once per bot before they get the message and start crawling the new site.

In any case, redirects from the old site to the new won't impact load time unless the redirect is happening all the time, which should never be the case in a site migration. Do make sure to get the right redirect rule and check that it's working, especially on your most-trafficked pages.

I think there's an underlying assumption here that duplicate content will harm your site, and that's not necessarily true. There's no "duplicate content penalty" - it's more than a filter. Google is better than most at recognizing this, especially with common CMS like Magento and WP. Google attempts to look at the links going to both pages and understand their authority together.

Duplicate content is more of an issue if you're pulling content that others are using as well, e.g. on product descriptions provided by manufacturers and other types of content. Google won't "penalize" you, but they will sometimes filter your site out in favor of the most authoritative site with that content. It's also an issue (mostly for Panda) if you're creating keyword pages that contain duplicate of even very-similar content just to rank for a bunch of very similar keywords.

So my first bit of advice is, "don't obsess over intra-site duplicate content."

That said, it's best to reduce and avoid duplicate content 1) for less-sophisticated search engine, 2) for the sake of your own analytics data integrity and simplicity, 3) just in case Google doesn't get it (very rare).

Set the categories up however you think is best for the user (generally just the product name without categories), double-check the canonical URLs, and wait for Google to catch up on the canonical and noindex. It can take many months depending on your site's authority, but it's unlikely to move the needle either way. Keep in mind that Google may keep pages in the index even if they are honoring the canonical tag - they'll just show the canonical version but keep both indexed. That's working as intended - don't worry about that

This is what I'd do:

• Use the brand name in the anchor text
• Allow people to remove them (or, to be extra safe, turn it off by default and prompt people to turn it on)
• Keep the links followed.

You could definitely debate the last point, but as long as you're not paying people for the link itself or trying to manipulate Google's algorithm it shouldn't be a big deal. If you're extremely risk averse, go ahead and add nofollow to the links.

I doubt it would have a negative impact, but I would make sure that the alt attribute accurately describes the image. The alt attribute is just a small factor in ranking, so I would not spend too much time on it

Just make sure that you're using the most appropriate image: if you're re-using the same image to target different types of keywords, you might be using images that depict something too broad. You be the judge: if you think it's a good image for the user, I can't imagine it getting you into trouble.