I suck at that stuff, your starter should be spinning pretty fast. If you know how to make a video we can tell you if it's fast enough. If you can't there's some videos of guys before and after starter swaps so you can get an idea of what you're looking for.
Yes, but that would only be with the big cable that runs down to the starter or with one (or both) of the battery ground cables. The smaller wires that are around the passenger's side fender area don't carry much current to the starter. Definitely not enough to turn the starter over and aren't for that purpose anyway.
I'm also no electrical expert. My guess would be that if you can set your multimeter to the "amps" setting, touch the big positive cable on the starter, ground the negative lead (a good ground), and have someone turn the key to use the starter, then you should be able to see how many amps you're getting there. I also may be wrong on that part so there's your disclaimer.
Yes if the headlight switch is bad or if the plug on the back side of that switch is bad. Sometimes that happens when your dimmer switch is bad too. At least on the ones with a foot operated dimmer switch.
Sometimes it's still extremely hard to tell on a video. It's part educated guess, part pulling an answer out of where you sit down. Other times, it easy to tell. Ky85's video was one of those times where it's very hard to tell.
That's the fuel return line. The fuel supply line is at the filter head, which then goes to the back of the injection pump. The injection pump does need about 6psi of positive pressure, which is generally provided by the mechanical lift pump. So if you bypass the tanks, tap the line at the lift pump inlet.
I'll take you up on your electrical disclaimer. Not ragging you but just going to be brief. Don't do this OP. First, you measure current in series to the circuit not parallel. If you did measure it properly in series you would blow the meter out. It's rare to have a current issue in a circuit, the meter setting is for finding small amounts of parasitic draw, with most meters maxing at 10amps. For your question, you would have to use an ammeter.
There's a compare and contrast video somewhere that I looked at recently, but Ky85s video was the only one I could find. And I thought it was a decent representation of "sounds weak". I think the other videos was on a thread about the "Powermaster" starter.
Next for your fuel, from what Nero said, pull the supply off of the lift pump which is on the bottom front passenger side and put a hose on the nipple into a container of diesel.
You’re right - The IDI’s can be stubborn.
