1. Many people. Team Twiizers made HBC, the main homebrew loader, and most of the exploits used to install it, but many, many people develop homebrew.
2. Most homebrew is legal. There are some "packages" which bundle illegal copyright Nintendo code, but those "packages" are only for enabling piracy.
3. Homebrew browser is merely a piece of homebrew which connects to the internet and downloads more homebrew. If homebrew is legal, so is Homebrew browser, so yes its legal.
4. Playing ANY game you don't own is illegal, regardless of how old it is.
5. C++ or C. I think theres a Lua interpreter too (Lua is a scripting language, so easier to learn than C++ or C, but needs an interpreter and isn't as powerful). HTML is not programming language, its a markup language.
6. Same way as any other developer's app: put it in the "apps" folder on your SD card. If you mean how do you get it on Homebrew Browser, I'd advise posting it on the WiiBrew wiki at [
www.wiibrew.org] and contacting the dev of Homebrew Browser over at [
www.codemii.com]