First of all, your questions are all based on misconceptions. Cubans, particularly those without access to tourist income, are possibly feeling some shortages of certain foods like eggs and chicken currently. Resorts and tourist paladars and casas that rent rooms to tourists are able to find and pay for food. Some restaurants have a deal with certain farmers who grow just for them. Lines for cash sales may be longer for certain items right now, and there are limits on how much each person can buy to avoid hoarding and resale for profit. Ordinary Cubans who have low level jobs and not a lot of "extra" cash, who need to supplement their monthly free food "basket", are probably the ones starting to feel a pinch on certain foods.
But at the best of times, IMO, you don't go to Cuba for the food. Eat Cuban food in Miami; if you want ropa vieja made with beef, not pork, and hamburgers, same..Salad is typically shredded cabbage, for example; tourists may get a couple of lettuce leaves at a fancier paladar. The seafood (shrimp and langostino/lobster especially) has been good at some of the upper level paladars or when we get it on the black market to bring home. Fish depends on where they get it, and where you eat it, from great fresh caught to thin, tasteless frozen fillets.

1. Most fundamentally, if things don't change much re the food (and perhaps fuel) situation, what should we expect in terms of availability of meats, fish, veggies, fruit, etc? (see above--If it doesn't change, you'll be fine.)

2. Should hygiene also be a consideration? I'm involved in international development work, so have some knowledge of food hygiene issues in some poorer countries. I know that Cuba's relatively high education and public health awareness could help mitigate this problem. But still, when food, refrigeration or electricity are scarce, particularly in a tropical climate, bad stuff can be cooked and sold.

Drink bottled water. Ice is made with filtered water usually. Havana residents boil the tap water and chill it to drink. I brush teeth with tap water, no problem. Street food stands that have running water should be People have refrigerators. So far electricity has been OK. During the hurricane, it was sporadic though in certain neighborhoods.
Take hand sanitizer and use it liberally..Bathrooms often don't have soap. Who knows whose hands were on your eating and serving utensils.

3. I don't have much historical context. How unusual is this new development? Was the situation after the collapse of the Soviet Union like this? Any similar situations over the past 30 years?
The Special Period was much worse than the current situation. There are always periodic shortages of something.

4. Any sense/predictions about whether, when and how the situation could improve between now and next February?
10 months out? Nobody can predict. IMO you're fretting about nothing, yet.

Edit: Here's a very good explanation of the situation on the ground by an expat living in Cuba.
https://www.tripadvisor.com/ShowTopi...tion-Cuba.html