normal to have a hot rotor after a ride. You CANNOT
use WD40 on brake parts. You stopped the squeaking but did not address the underlying issue behind why the squeak is occurring.
There is an issue with your rear wheel alignment, whether it's a missing spacer, improperly seated dust seal, sticking piston, or issue with the pins.
You need to remove the rear wheel, thoroughly clean off the rear rotor with red Scotch-Brite, check that the dust seal is seated properly, make sure the correct spacer is in both sides of the wheel, install new pads, rebuild the rear caliper, clean and lubricate the mounting pins and bores, and rebleed the entire system.
amounts of ceramic brake grease can be applied to the contact points of the metal bracket that holds the brake pads. WD40 will spread beyond where it is applied and has contaminated your rear brake.
Look at the seal on your bike and compare it to the picture below. Your seal is not seated and is right up against the caliper mount bracket.