Regarding the reversing section that needs to be isolated and powered through an autoreverser, you want that to be longer than you longest train, which usually means as long as possible on small layouts.
I would isolate the diagonal track in the picture, starting right after the turnouts coming off the loop tracks. That would include the 2 sets of spurt tracks coming off the diagonal track.
In use, you will not be able to switch the industries on those two sets of spurs at the same time from the 2 different directions, because you cannot match the loop rai phases at both ends at the same time.
But, the track pattern makes it look like the intent is to come onto the layout from the upper right corner, running the loop counter-clockwise, then take the diagonal reversing track, switch the bottom of the picture industries on the way through the reversing section, then go around the left side of the loop and switch the upper industries, then continue clockwise until exiting the layout at the top right corner. So, no need to have trains on both sets of isolating rail gaps at the same time.
