Vertical trajectory concept applies to the pumping (yo-yo) mode, where the tether length varies as for all yo-yo systems. To my knowledge, it has not still be tested. References are given in the preprint. The main explains are on the publication [3], Chapter 13 of the second AWE book:
Milan Milutinović, Mirko Čorić & Joško Deur: Optimization-Inspired Control Strategyfor a Magnus Effect-Based Airborne Wind Energy System.Roland Schmehl. Airborne Wind Energy. Advances in Technology Development and Research, Springer, pp.303-333, 2018, Green Energy and Technology
https://www.researchgate.net/publication/324134888_Optimization-Inspired_Control_Strategy_for_a_Magnus_Effect-Based_Airborne_Wind_Energy_System
PDF available on request.
Then page 2 of the preprint:
A vertical trajectory is visualized on the Fig.12.23 [7] in the end of the publication.
Reference [7] of the Chapter 12 of the same book:
Ahmad Hably, Jonathan Dumon, Garrett Smith, Pascal Bellemain: Control of aMagnus Effect-Based Airborne Wind Energy System.Roland Schmehl. Airborne Wind Energy. Advances in Technology Development and Research, Springer, pp.277-301, 2018, Green Energy and Technology, 978-981-10-1946-3. 10.1007/978-981-10-1947-0_12 hal-01759173
https://www.researchgate.net/publication/324134958_Control_of_a_Magnus_Effect-Based_Airborne_Wind_Energy_System
PDF available on request.
That said this chapter is also available on https://hal.science/hal-01759173/document. The same Fig. is Fig.1.24, representing a vertical trajectory. See also the conclusion including an estimation of the power/m² (1.48 kW/m²). In my preprint I did not use this estimation, preferring to base myself on Omnidea’s experiments, then extrapolate in vertical trajectory, based on the Chapter 13. As a result the spin ratio is far lower, as for the power/m² (about 400 W, roughly as for the Mutiny power kite [reference 4 in the preprint], assuming a same wind speed of 10 m/s.) I think high spin ratios above 2 are not achievable due the power consumption by the cube of the tangential speed of rotation.
In few words I think (but explains are better in the chapter 13) the vertical trajectory far from the winch (as noted in the chapter 13) is more efficient than the basic oblique downwind trajectory, because a crosswind component is added, and the balloon does not go downwind. It is a little (but not quite) as if you fly a power kite vertically in reel-out phase.
Yes (no reference in the preprint), crosswind eight-figure trajectories were studied:
Modeling and control of a Magnus effect-based airborne wind energy system in crosswind maneuvers
PDF available on https://www.researchgate.net/publication/320495787_Modeling_and_control_of_a_Magnus_effect-based_airborne_wind_energy_system_in_crosswind_maneuvers
If I remember correctly, the numbers given seem to lead to a power/m² being 2 times higher than for vertical trajectory. WindFisher uses this crosswind trajectory.
Yes: I prefer the motors are close to the balloon is such a way the respective belts are sufficiently tight all round. That said launching a heaver than air tethered Magnus effect balloon can be difficult or even impossible due to stability issue: in flight, perhaps the weight of the belt motors below the balloons can improve stability, but this is a point to be verified. If electric heating appliances are added in the ends of the balloon, combined with an insulating envelope, some neutral to light buoyancy can occur. But all this has to be studied deeper. Hydrogen or helium should be avoided.