2. Why not build Great Library? Why is it bad?
The Great Library gives you technologies when you don't need them. If you are interested in getting to a specific technology, the Great Library has the potential to slow you down because each technology you get increases the cost of researching other technologies.
In Civ 2, there are 3 "technology cycles," and each technology is assigned to one of the cycles. Except for scenarios (and I think the Chieftain level), every time you select a technology to research, the technologies from one of the cycles will be hidden. This means that as you try to get to a certain technology, you will sometimes have to research technologies that are "off path." So, for example, if you have to research an off path tech and then you get 2 unneeded techs from the GL, you will then have to research off path again.
If you go to the trouble of planning out when you will research technologies, the GL will ruin your plan with any unwanted technology.
Sometimes, you can even get a technology that will hurt you. The best example is feudalism, which makes the warrior obsolete. If you are in Monarchy, you won't get the benefits of a cheap martial law unit, and the warrior is nice to have for cheaper Incremental Rush Building/Buying (look it up, or ask about it).
Also, if you play Classic, you can use the cheaper Marco Polo's Embassy to trade for a technology that only one other civ has.
4. Ok, caravans are good, but what do you gain from it? Commerce or science? And does it cost anything but production?
When you deliver a caravan (or freight) you will immediately get a delivery bonus. The game will tell you that you got a certain amount of gold, but you will also get an equal amount of science "beakers" added to your research.
You will also get an ongoing bonus of some extra trade arrows in the participating cities.
Caravans only cost the 50 shield production, and the ship and time to take them where they will make a profit.
Note, don't send food caravans don't pay money, but are used to move food around. I usually use them when I earmark a caravan for contributing to a Wonder of the World.
3. Oh, how to get the non-hostile AI patch?
I don't use it, so someone else will have to answer. I spent money and bought a copy of Windows XP 32 bit so that I will be able to virtualize the environment forever.