yes
There are many methods, but, the main one is to use a game launcher and click download.
They don't. Only games distributed and managed by Steam can make use of the Steam API.
Yes you can, you import and add non-steam games to your Steam library. From within the library you can launch and run any games, although 'Steam achievements' will only be available for Steam enabled games.
A simple search of the net will find there are many bandwidth monitoring sites that are available for free. A popular one is callled Free Meter Bandwidth Meter. You also can use WFilter freeware for network bandwidth monitoring.
If I understand your question correctly, you are asking if playing games (such as flash games) use a high amount of bandwidth. This depends on what kind of games you are playing. A flash game, Bloons for example, would use considerably less bandwidth than a game like Oblivion.
Yes you can.
It is an addon that you buy from steam to use with other games to give them more features.
java
Yes
If you don't have unlimited bandwidth then yes!
The use of the Linux bandwidth monitor is the way to properly monitor network traffic with reports. They use these to know the bandwidth unique usage.
This is possible, but only if the game already is available to buy from the Steam store. For example, if you have Half Life 1 from back when it was not on Steam, you can validate the CD key on Steam...and then play it without the disc through that. Games that Steam doesn't support however can not be translated over to Steam. Quite a few games are on Steam though, including EA recently adding a number of their games to it.