Yes, of course. It’s not tri-mode.
The original 45drives pods, i believe, had forward breakout cables to each drive. There is a thread or two here from forum members rebuilding old pods. There isn’t any downside to the backplane that I know of, there is still a direct data path from each drive to the motherboard/HBA. Some backplane designs from server manufacturers use port multiplier designs that multiplex the data from multiple drives over one cable. For example, I have a Supermicro server that has 16 drives running off of just two SFF-8087 connectors. There is, as you would expect, a performance hit with that design when under heavy workload. As I described, the HL15 backplane is not a port multiplier backplane.
For power disable, see;
https://forum.45homelab.com/t/hdd-power-disable-feature-pin-3/404
I’ll let Hutch or someone answer about the power distribution.