#2
Most guitars with agathis bodies aren't too great but I reckon it depends on the quality of the woods anyways. Not all examples of wood are 'good' tone woods. You can get turd pieces of mahogany and the like.
#3
It's usually not too good and mainly found on beginner's guitars, but the quality varies. I have seen it used on mid range BC Rich guitars, too, so it seems it's not only for entry level instruments.
I have an old Ibanez made of Agathis and the sound is not too bad. It lacks in the bass department and has lots of highs and upper mids, kinda thin sounding, though the pickups make up for it.
If you have to go cheap, Agathis is okay, but if you're foing to spend 200$ or more, you can already get basswood or alder sometimes.
#4
Some will say its no good. The reason it seems to come in lower budget guitars is low budget guitars come from asia where the tree grows. Its just economics. Alder is relatively cheap but doesnt grow in asia so it would have to be shipped therefore increasing the price. As many want a "cheap" guitar agathis is a way to decrease the manufacturing cost. For a long time alder was considered to be cheap wood and therefore no good. Its not bad wood just low cost.