(1) Yea, sounds interesting
(2) Perhaps, but would be heavily dependent upon when
(3)(A) Google Hangouts is a good choice here. I have personally been in multiple 5-6 person video chats and they worked well. As with any technology, there is a "startup cost" to get people initially familiar with it, but after that it runs smooth.
(3)(B) Is it 20 or 50? I ask because this is - in my experience - significant. There are a number of the solutions that start to break down at about 25 people, especially when there is video or screen sharing.
So I think you should think about what precisely you need. For example, if you are expecting this to be just a "broadcast", then YouTube Live or some equivalent might work (and you can project Google Hangouts to YouTube Live to get scale - Google for "Hangouts on Air").
The gap to what you have described then becomes the chat/questions - and how to draw those in. Most times when I have done webinars, they have been recorded, so we are careful about how questions come in. If it's all company-internal, we use an operator and allow people to speak. If it includes random Internet people, then it's text only, so we can choose what audio is permanently recorded. This avoids pitfalls of open microphones for un-validated people. In a rare case, we used closed registration and allowed open mic, since we reviewed the attendees in advance to know it didn't have competitors, media, et al. It's easy to think everyone showing up will be nice - and in general that will probably be the case - but you also know how comments can go in news articles about FIRE :)
One other point on it coming back to what you want to have happen. If you want the audience to be able to interact with each other, then that may provide different requirements than if it's just question gathering. If it's just gathering, you could use Hangouts/YouTube have someone acting as a gateway and then asking the question of the rest of the "panel of presenters". They could even be monitoring many forums (Twitter, some chat room, et al) to pull in questions. If you want the questions to potentially be answered by other participants and answer good FAQs or hard questions "on air" (usually how it happens at my work), that may require more of a formal webinar software (which tend to be pricey)... but that is also a scenario we only use internal to my company... the external stuff is all questions only to the presenters.