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Two servers? The existing one to process the redirects and a new one to handle the reverse proxy. Or vice versa. So the DNS for the old domain would point to a server that does the redirects. However, the server that hosts the site will be set to reverse proxy. Another way of looking at this concept would be to take down and redirect the old site, and to start a new site, with the exact same files/database, that will be used to serve the content in the subdirectory/folder. Ask what they think of that idea. I certainly don't have all the answers to every situations, but will do my best to help you find a workable solution.

Hi Jacob, Use subfolders. Remember to use the hreflag tag, inclufing the country code. If  you have the ccTLD domains, redirect them to the subfolder. For example: If you have yoursite.co.uk point it to yoursite.com/uk/ Also, remember to add every subfolder to Google Search Console (Google Web Masters Tools) and declare for each one the country that is itended to. Hope it helps. GR.

Not only can you but you should. Pruning non-performing pages that you don't intend to rework and get ranking is very useful. I would definitely 301 the pages though, even if you 301 them all to the homepage (not really recommended if there's any better option.) Get a full site crawl with Screaming Frog, your sitemap or some other tool, import to Excel, add a column for your new URLs (homepage or other) and then just concatenate them into a long list of 301s. Or do some wildcard magic and make it simpler. Depends on the url structures for that, I suppose. But yes, I would definitely redirect and get rid of the tens of thousands of non-useful pages. It should actually help your other 100 as well.