Yes, those Johnny comics are tests, probably of the two-panel comic functionality but possibly also of the comic editing functionality. In the olden days they used to have three panels each, and let me tell you, they were a lot funnier when they did.
Yes, the Life in Hell character is Bongo. Life in Hell was around long before the Simpsons, I think, so it's probably the company that was named after the character instead of the other way around.
I'm given to understand that Brad doesn't like the legal headache of having other people's copyrighted work on his site, even with the limited permission to use it, and would use only non-copyrighted work if he had it to do again. So I wouldn't expect to see much in the way of art from webcomics in the future, unless I'm mistaken or Brad has a change of heart. That's probably sensible, but on the other hand, I imagine that the idea of making comics with characters they already knew was what drew some people to this site in the first place. Not me, but some people.
I should probably tease you people with nonsensical jokes like Big Evil Dan does, instead of giving you straight answers, but it's so hard to resist flashing my old-school credentials. Anyway this isn't the first time I've played historian here. Dan is old-school too, though. The quote in his sig is from Three Reasons.
Did you kids know that the art in our Penny-Arcade category is not, strictly speaking, from Penny Arcade, but from a long-defunct PA side project in community cartooning called The Bench? It was sort of like Stripcreator, except that the art selection was much more limited (even in those early days), and you had to assemble the comics yourself in Photoshop or the like, and only hand-selected comics got put on the site. I never participated in it or paid much attention to it, but I think I've got the basic concept right.