I have an update. To see if I could troubleshoot this some on my own, maybe find the point of failure, I installed a fresh version of AbanteCart and went about building the store as I did before, checking the front end every few steps to see if the scripts I mentioned before were still working.
I deleted the sample customers, orders, products, categories, and manufacturers, and then created a product. No problems up until that point.
Next, I modified the page layouts for "Default Page Layout", "Home Page", and "Default Product Page", and then uploaded a new version of "style.css" via FileZilla (version 3.9.0.5) and checked those scripts again. They'd failed, just like they had before.
I restored the default version of "style.css" and returned the page layouts to as close to their default appearances as I could remember, but the scrips still did not work.
I did not modify this new test install of AbanteCart in any other ways than those I just described.
Any idea what could be going on?