Like anything in SEO, there isn't an exact answer. It's partly because Google shows titles of different lengths depending on what size screen it's showing on. It's also because characters can be different pixel widths. For example, an "i" is not very wide, while a capital "M" or "W" is a wider character, just by the nature of the letter itself.

Yoast openly states that the optimal title length depends on what kind of result is going to be shown, so their advice is to make sure your keyphrase is within the first half of your title and not stress about exact character count or pixel width.

Moz keeps a close eye on this as well, and their research concludes that approximately 50 to 60 characters, or 600 pixels, is the optimal length, and it will show on about 90% of search results. Moz Pro reports that anything over 570 pixels is too large, though, so if you're under their 600px recommendation you can still get the warning that your titles are too long.

All things considered, as long as you're not grossly outside of either target - 60 characters or 600 pixels - I'd take it with a grain of salt and consider just marking "ignore" on the titles that are close to tolerance. There are no hard and fast rules - except the rule that Google will continue to change - and we may see shorter or longer limits in the future. Tweaking your titles to exactly within both tools' tolerance is probably not going to get you the best return for your time - see what other more high-priority issues you have, like 404 errors or missing meta descriptions. (Change case in point, Google just changed their meta description length maximum.)

It's more of an accessibility issue than an SEO issue. If your visitor were using a screen reader, would it add any value for them to hear the alt text? If so, keep it. If instead you're describing "curly swash with a heart" and it has no bearing at all on actual content, leave it blank. Generally speaking, things that improve your accessibility are good for SEO.

It sounds like part of the problem may be the sitemaps you're sending. By including duplicates in a sitemap, you're basically telling Google that each version of the page is valid. I would remove them and resubmit a sitemap with only the canonical versions you want indexed and see if that helps.

1. Which filters you use really depends on which filters apply. This shouldn't really affect SEO at all. Just focus on what will be easiest for your user base.

2. This could potentially cause an issue, but depends on how it is implemented. More info straight from Google: https://webmasters.googleblog.com/2014/02/faceted-navigation-best-and-5-of-worst.html

3. If you don't sell retail products, don't include "wholesale" in the URL. Google and users prefer shorter URLs. Don't put them in the product titles or slugs either (slugs being the last part of the URL, i.e. /products/my-product-name = my-product-name is the slug).

4. Couldn't you put the SKU right on the PDP, so if employees search by SKU they find the correct product anyway? Seems like a lot of extra overhead setting up 2 URLs for every product and then redirecting and canonicalizing everything. There's a lot of potential for mistakes.