1. I keep (a) betta and know a little bit about them, I'm not an expert by any stretch but I know a little bit.
2. Other methods for what? Breeding them?
3. Personal opinion. Some people are happy to keep them in cups or jugs, some like to keep them in little 2.5gal tanks, and some (like myself) won't put them in anything smaller than 50ltrs (approx 12gal) for the long finned ones and 100ltrs (approx 25gal) for the short finned ones. But as I said, its all personal opinion. If you feel like your betta is happy in a cup, then I guess he will live in a cup.
4. They do not need live food to be healthy, a good brand of pellet (New Life Spectrum for example) will keep them perfectly healthy. It may help to condition them with small live foods for breeding but I don't think it would be "essential", they will still breed if fed pellets.
5. Both of mine lived in community tanks and I never had a problem with them but it all comes down to the individual betta. I have a friend who's betta won't tolerate any other fish in his tank no matter how bleak or mundane the other fishes color. Mine on the other hand happily shares his tank with every fish I've put him with.
If you are going to try one in a community I advise making sure the other fish don't have long fins or bright flashy colours as the betta may get 'jealous' and try fighting them. Also make sure that none of your other fish are nippy as the long fins on fancy bettas are very likely to get nipped.
I've had bettas in community tanks ranging from 120ltrs to 250ltrs, I've not tried in anything smaller than 120ltrs (approx 30gal) so I don't know how well that would work.
6. I couldn't tell you if 850ml jars are big enough, personally I wouldn't do it, but I'm sure it probably could be done.
7. I would assume a larger tank with little sections would probably be easier to maintain the water quality in than a bunch of jars, and that way you only have to do one water change on one tank as opposed to emptying and refilling hundreds of smaller jars. Just make sure the males can't see each other through the separators otherwise they'll spend all their time flaring up at each other which I'm sure can't be good for their long term health (I don't know this for sure but it seems like it makes sense)
8. I don't know about paradise fish or gouramis, as I've never kept them.
9. Yes. Go into your lfs and have a look at their set up, I'm sure if you asked nicely the staff would show you.
As I said I'm not an expert by any definition but I hope some of this information is useful to you.