You raise relevant points but not without possible answers for what I think.
Gravity is a disadvantage during the climb (reel-out phase), but an advantage during the descent (reel-in phase), considering vertical paths.
Both chapter 12 and chapter 13 mention quite vertical trajectories by taking account of reel-out and reel-in phases. The tether reel-out speed results from the increasing of the elevation angle of said tether. During reel-out phase the kite undergoes both the same real and apparent winds in a way that the angle of attack stays the same for what I think.
Yes, but the kinetic energy of the kite is used when the kite ends its raising where the power zone decreases by going towards the zenith. As a result the loss of time to accelerate is compensated by achieving a higher altitude and elevation angle.
Yes, but turns are needed and require place, preventing the kite scaling up beyond some point, unless it is divided into smaller unities whose the management can be quite difficult.
Or even only one kite as for Low radius loop - #7 by PierreB.
It depends, among other things, on how far the kite can scale. The purpose is the same: maximizing the space. For what I think the figure-eight does not allow it enough due to flight requirement.