I don't trim any fat from between the muscles. I've seen videos where they do. Bad idea I think. Most of those videos trim way too much. I don't think trimming between the muscles is necessary (of course, I separate the point and flat because of my #1, read on). I just shave down that really huge glob on the top to about 1/2 inch. If I'm being really fussy, 3/8 inch fat is the absolute thinnest I will go. Score the thick areas of surface fat, it will help them soften and render, without drying out the meat.
That was probably me who recommended the electric knife for brisket. I also have a 12" Victrorinox Granton Edge slicing knife, which is awesome, but for tender brisket, can't beat the electric. Franklin used an electric to slice his brisket for competition bbq, so I'm not going to be ashamed of that.
I have had GREAT results with whole packer brisket on my #1. To fit, I need to separate the point from the flat, and smoke on separate racks. Everyone here says it is best to smoke in one piece...but sometimes I wonder if separating gives me some advantages. I put the point on the bottom rack fat side down. Flat goes on the top rack fat side up. The point on the bottom, which can handle a little more heat, protects the flat on the top rack from direct heat. This is also a good reason that I leave plenty of fat on the top of the point (which is facing the direct heat from the element/wood box). The point always seems to get to 200-205 at almost the same time as the flat gets to 195. Also, it is my theory that the internal size of the smoker to meat mass ratio seems to be almost ideal for maintaining box temp, moisture, space around meat for smoke, etc.