Don't translate proper names. Especially things like New York, I would be willing to bet that everyone has heard of that. But I take the stance to not translate proper nouns, it dilutes the word and causes confusion.

Actually have recommended products on page, or other products in the same category. You could have it pop up a fancy / color/ thickbox when people add to the cart and recommend other products, something like this http://module-presta.com/media/catalog/product/cache/1/thumbnail/9df78eab33525d08d6e5fb8d27136e95/i/m/imgforuserguide_popup_cart_02.jpg

There are all kinds of different things that you can do to cross sell. Look at what amazon does and how they operate, they optimize the product pages. On them they have lots of different product offers.

Honestly in my opinion you are going to have a hard time making the category rank higher than the product. Especially since things like rich data only supports one offer per page. I have no proof, but I would off the hip say that search engine want to put people on a product page, not a category page.

One thing to take into account to is conversion rate, just about every study says you convert more with a product as a landing page vs any other page in a site.

This comes down to personal preference in my opinion. I think honestly it is neither here nor there. The instances that your 404 page could come up in the SERP's is more than likely pretty low and at the same time it really does not offer any useful content. So disallowing it would not really be any loss. One reason why it might be disallowed is that you have an e-commerce site that rotates products. When a product is deleted, the developer has a 301 to the 404 page, then with the robots.txt saying not to index the 404 page, the other page will drop out of search engines. If this is the case I would rethink that strategy. If you notice a lot of sites like amazon and other big sites leave the page in the index even if the product is no longer for sale. The thought is traffic is traffic, the hardest part in the whole equation is getting someone to your site, if the page is ranking, why delete it.

The only time I can think that I would specifically allow it and optimize it is if you have a cool 404 page. Some companies actually spend a bit of time on their pages and it gets them a little pop of viral traffic from social sharing sites like reddit. If you do have one that is funny or unique I would allow it and actually optimize it for a term like "funny 404 page" or something like that.

I would host on both. It seems to be what SEO Moz does, so I am sure that is the best strategy.

Check here

The easiest thing i can think of is to write a script that works with your dispatcher to create a site map. The format I would use is add the page and all of the "product images" on the page to the map and move to the next. At the same time I would use an auto increment variable to keep track of how many lines you have written. When you get around 50k, write out the name of the next site map file that the program will create and have them chained together this way.

Are you using a custom platform or an off the shelf e-commerce package? Most off the shelf packages actually have a module that can create a site map and a lot have it where you can cron it too.