It would be very helpful to upload a ZIP file with resources (e.g. PDF, images, etc...) and have AbanteCart automatically unzip the file and add the individual files to the product.

That's a difficult question to answer, since the "speed" of a web site is dependent on many factors, most of which are out of the control of AbanteCart. My suggestion would be to install AbanteCart on a test server and try it.

I visited your site, it wasn't that slow for me. I had to wait a bit for a few pages, but it was otherwise pretty snappy, and I am in the US.

I don't think so...the closest thing available is https://abantecart.atlassian.net/wiki/spaces/AD/pages/15466660/Language+Definitions.
The idea is you can look through the Language Definitions in Admin Console > System > Localization > Language Definitions and make your translations there. Most (not all) localized strings can be found there.

What happens if you delete the content of the template entirely and leave it very basic, does the e-mail send then?

Is there anyway to turn on the Search in product model and Search in product descriptions functions by default for all searches?

This seems like a significant security hole...

I am wondering if a trial period is available for the Elasticsearch extension? It is an expensive extension, along with Elasticsearch itself so I would like to try it out, if possible before deciding to keep it.