Saw the post this morning here and over on brand x(hometime). It was part answered there and Jim finished up. I was too busy today to get into a lengthy answer.
Couple things though. If you have a short piece of 14/2 around use it for a short run to a light. If buying a roll, buy 12/2. Except on strictly low bid tract houses, many electricians I know are putting in 12 guage for a few reasons. Doesn't cost that much more, save having to pick from different rolls of cable, more and more circuits are being required to be 20A, allows for expansion on a circuit later(especially with all the new electronic stuff coming out), and mainly cause whenever a homeowner has a breaker problem, they replace the 15 with a 20 and then you have a potential fire situation.
Also a double switch may save getting a double gang box, but don't forget your cable fill requirements. Adding another cable to your box may overcrowd it causing a heat and fire problem. Three 12/2 cables going in and exiting a box require 20.25 cu.in. Make sure you have the right size box for the number of wires or it is a code violation. Two switches require a double gang box and adding a humidistat would require another gang to what ever you have. Caveat=don't scrimp and pick up cheap boxes. Code requires bigger boxes for more cables.
Jim is right since you don't have a switch leg, all white/neutral/grounded conductors go together, all bare/green/grounding wires all go together, use 6 to 8" pigtails from the black/hot/line conductor to go to the switches. If adding the humidstat add another pigtail from the hot to the humidistat. Then from the load side of the humidstat and fan switch use pigtails that connect to the black wire going to the fan so that way either one will make it work at the same time.
Use the right size wire nuts and most electricians twist the ends with pliers, snip the end off, and then add the wire nut. Also one wire per screw. Don't use the push in terminals(too many problems with them). Always run a hot/line cable into the same place on every box so you can keep track such as the top left slot. Same for which screw to use, run the black/hot wire to the upper screw on a switch or receptacle so you can always look and know which one is hot(with preexisting wiring do it the same as the electrician who wired it did). Line side comes from panel and load side goes to fan/light,etc. Good luck.