Wasn`t sure if I should reply here or on the sister site, here looks like the place.
In your shoes, I`d have gone the old fashioned route and just called them. There are so many little details that could be covered more easily in a phone call than emailing back and forth. Like Don above, I`ve had fantastic luck speaking with them any time I`ve had a question or warranty concern.
If I`ve got a tool that`s not performing as I expect where it may be a warranty concern, I`d much rather discuss it in person. Especially in your situation where it seemed like you were on the fence of even just using their 180 day satisfaction guarantee and returning it (not sure if too much time has passed now?).
I`d forgotten about your TBI, so I understand where that could come into play for preferring to email.
As far as the brush situation, I`ve heard of people in the past with new polishers (not just Griot`s) where the brushes may not have been fully seated from the factory. Your G9 came with an extra set of brushes so you won`t need to wait for ones to show up in the mail. Also, you`re dealing with a high power brushed electric motor - the area around the brushes is naturally going to get warm. If you try your other set and don`t see a change, then I would contact them about sending your unit in to have them look at it. That will remove any question of "Is this normal or not".
Just to check - what backing plate and pads are you using? They talked a lot when the G9 was released about how the backing plate is a big part of cooling with the tool, so just wanted to make sure you`re using either their black vented 5" plate or the 6" plate that came on the machine along with a pad that has a hole in the center.
Sorry this has been a negative experience for you. I absolutely love my G9. Mine was from nearly right when they released them, so I did run into the glitchy power cord (the unit would just not work unless the cord was flexed just-so), but other than that it`s been a champ. It`s done multiple 2 stage jobs and I`ve not noticed any heat issues. But I`ve also not done much gripping around the brush area with all the other ways to hold the machine, as well as just being aware that`s where 1000 watts of electrical energy gets converted to kinetic energy.
EDIT - One other thing would be that I would call during their normal work day - While I think their support line is 24 hours a day, during the work day you`re more likely to get one of their seasoned on-site people. I think I`ve read where when people have called off hours it sounds like you get a different crew that may be more focused on being able to take orders and answer simple questions vs answer deep product questions. This is purely my speculation, as I mentioned above I`ve only called during the work day.
