Using 2x CAT6 cables for 3D Printer Hot End
At first thought I blew off CAT6 as a viable option, but thinking about it #26 Wire is not a bad choice.

So to answer your questions:

You should have the same number of power and ground lines in parallel for power delivery like this. \$ 50W/24V= 2.1A\$ Using one of many online calculators for voltage drop, assuming 1ft cable of #26, your looking at 0.7% voltage loss (~1/3W per foot, starting to get warm). 2 cables in parallel that is 0.35%, not to noticeable. (example here) I would suggest using 2 twisted pairs, with power and ground on each pair to reduce inductance and loop area.

Q1&amp;2: #26AWG cable is fine if you use 2.

Q3: Depends on the amount of power, and how your control board works, if anything uses low side (NFET) switches to control fan speed etc, then common grounds breaks that scheme. Any fan etc. that has speed control I would send independent wires, unless you are sure of how it works.

Q4: Yes, mixed voltages are fine.

Final note, you should try to keep things paired with Ethernet/RJ45 pairs.
i.e. run a fan on a twisted pair, don't pick random wires and mix, lookup what pins go to what pair.