Just curious, what were you feeding the birds prior to processing them? I had 6 males processed last fall, they were about 7 months old...none of my birds had the amounts of fat you described, there was a little within the cavity, but not much. Guineas are lean by nature... I ended up leaving the skin on, because they looked so lean... I did not want to end up with Guinea jerky, lol. The next batch I have processed (or process myself) I would like to have a little more fat on them to help keep the meat more moist.
Pithing the brain up thru the roof of the mouth prior to cutting the jugular helps with the feather plucking (with or without scalding).
And letting them "rest" 2 days in the fridge before cooking them helps make the meat more tender, especially in the older birds.