"Few" and "many" don't have precise definitions because they are both somewhat dependent on the overall group size. But you can think of them as roughly logical opposites, so when you negate "few" you can change it to "many". Here's a more in depth discussion about quantity terms like "few" and "many" from our blog: https://blog.powerscore.com/lsat/bid-15 ... -and-many/
To be honest, I wouldn't focus as much on the diagramming when trying to figure out why answer choice (A) is correct because, as you've discovered and as you can read in the complete explanation at the very beginning of this thread, it gets a little convoluted. From the premise chain, we know that if it is an environmental problem that is not the result of government mismanagement, then the changes need to be economically enticing. The conclusion then tells us that if many (or not few) serious ecological problems are solved, then the solutions must be made economically enticing. We need to connect this idea of the quantity
of ecological problems that must be solved with economically enticing changes to the idea that environmental problems that are not the result of government mismanagement must be solved with economically enticing changes.
That's what we can prephrase--if the premises are specifically about environmental problems that are not the result of government mismanagement and the conclusion is broadening that scope to all ecological problems, we need to basically say that few serious ecological problems are the result of government mismanagement. We're linking the concept of the amount of all ecological problems to the concept of ecological problems that are not the result of government mismanagement. The premise tells us that economically enticing solutions are necessary for a specific subset of environmental problems; the conclusion makes a leap to saying it is necessary for many serious ecological problems. So we need to show that many serious ecological problems fall into the category of "environmental problems that are not the result of government mismanagement."
Hope this helps!