First always remember that your car will have more connections that likely originally used. Saves money to make one or two wiring looms with ever possible connection than create multiples specifically for all the possibilities and just those ones
First picture purple wire and connection is for the brake warning light. Often owners/builders leave it off when they can't figure out how to center the piston that sits between the front and rear brake passages
Other black plug with the four wires is for (sorry don't have a wiring diagram open at the moment) the rear turn indicator lights for a Mach I with the fake hood scoop. Not used for your applciation
Second picture - The white boot goes to the carburetor idle solenoid your car came with originally. Keeps it from dieseling
Green wire with boot would be not used on your application since its for AC
Blue with red tracer/stripe wire is for a light - in this case believe its the one in the shifter bezel for an automatic - Again not used for your applciation
Absolutely correct. However, the green wire with boot was only available if (1) the car had AC and (2) the underdash harness is original to the car.