Starting with #3:
"3) Looking at travel insurance, what happens if husband or wife tests positive for COVID? Is only that person's coverage invoked? Or can one actually get the entire price of the cruise and airfare paid by the insurance? (I've never had to deal with travel insurance before)"
For all of our many insured cruises, and two were expeditions to Antarctica, the terms and conditions of the insurance would provide coverage if one of the couple became injured or ill pre-cruise and thus canceled the cruise. There was no expectation that the healthy partner would cruise. Covid was treated as any other illness that would preclude travel. Be aware that the insurance usually does not cover 100% of costs incurred. Recommend you visit the travel insurance board on CC:
https://boards.cruisecritic.com/forum/499-cruisetravel-insurance/
and that you pay a visit to https://tripinsurancestore.com/
for specific answers to your questions.
"2) Are we fools to book at all with a cruise line offering no consideration for Covid risks, while forcing us to be exposed to a greater risk of possible infection via a forced night's stay in Buenos Aries?"
No. Prepare for covid risks by purchasing a travel insurance policy that meets your needs and covers other potential risks. The burden is on the traveler to protect their financial interests.
"1) Are we fools for paying so much, when we could seemingly go twice for this price in a less elegant ship, and with less-esteemed naturalists? Absolutely everything is included, down to "open bar" and all gratuities, but still... pricey. "
No. It's so easy to compare numerous cruise lines that do expeditions to Antarctica. Do the research, put together a spreadsheet, and figure out the best expedition for you. Our first expedition was on the Marco Polo (now defunct), an old ship with a fantastic expedition staff that was led by Peter Hillary, son of Sir Edmund. Second expedition was on Seabourn, a luxury experience and great in a different way.
One thing to note: the expedition leaders from all the cruise lines tend to know each other and support each other. They share info between cruise lines about ice conditions and weather.
Antarctica can be the adventure of a lifetime. If you go.