John, much as I like and respect you, and appreciate all the good content and advice you provide here, I'm afraid I must disagree on a couple of your points.
They ran a single hot wire up the mast connecting the various loads (light bulbs) in series to the hot lead.
I have never seen this. Perhaps in parallel, in series you'd drop voltage across each load.
One switch all lights on the mast come on.
I've never seen this, either. Doesn't make sense for nav, steaming, foredeck, spreaders, etc.
follow the suggestion to use the existing wires to pull new properly colored wires up the mast.
Not usually possible, in my experience, given how wires are often secured inside the mast: in tie-wrapped bundles, etc.
You only need a single ground wire (now usually yellow - or old school black) if you are planning to use LED lights or lightbulbs for a multi light fixture on the mast head.
Not recommended. Especially with nav lights, which require a 3% max. drop. Good practice is a separate power and ground wire, all the way back to the panel, for each load. I know many (most?) boatbuilders cheat on this, but that's the way it should be done. A reasonable middle ground (pun intended) is a ground bus at the mast step, and a hefty ground back to the distribution panel.
I wish the ABYC would publish an inexpensive, or even free handbook for mariners like us, like "ABYC for Dummies." I admit, I'm a dummy, too. I don't have the money, time, or retention to read and remember the entire electrical standard.