Hi!
Thank you very much for the proper details, it helps me a lot!
1. I think I found the problem and fixed it via FTP. It took me some time to figure it out, there was a mixup with the term IDs. Can you please confirm that the correct language results are returned?
2. Yes, that gives the best possible SQL performance, with the least overhead.
3. Yes, check out this section of the documentation: Magnifier, return button actions
4. Well, a custom code might do the trick. There is a knowledge base article that might help you out with this one: https://wp-dreams.com/knowledge-base/woocommerce-add-to-cart-button-for-vertical-and-horizontal-results/
I hope this helps!