1) There are several manufacturers selling plastic bases. Chessex comes to my mind. But maybe someone else can chime in here. I bought a huge lot of plastic bases at a convention a few years back and since I don't paint units they will last for a loooooong time ...
2) Where have you seen CSO diorama miniatures with round bases? This year most, if not all, minis were on hex bases. Either regular sized or the bigger sizes available from IWM. I know my minis were.
I 'only' collect miniatures and don't use them for play. Miniatures I post to CSO get based on a hex base though. It is the 'correct' base for the game and that's what we want to show on CSO. I do have a few miniatures that are based on display plinths. Usually miniatures I paint specifically for competitions get based this way (
this one for example). We have been talking about showing miniatures not based on a hex base on CSO and you will probably see miniatures based on a plinth there soon.
Ultimately it is up to you how you want to base your miniatures. Round bases are not cheaper than hex bases. Maybe more readily available, but getting 100 hex bases from Chessex should solve this problem. If you want to play the game and don't necessarily want to play on a hex base, I recommend mounting the minis on hexes. Otherwise do what you feel looks best ;)
3) Green Stuff is arguably the most widely available epoxy putty. You should be able to get it in almost any hobby shop, online or offline. There is a huge variety of putties out there ... what do you want to do with it?
4) I keep them in their original bottles. If pressed I'd even say I prefer non-dropper bottles, but honestly I don't much care either way. Paint ratios are overrated in my opinion and for the most part not necessary.