Less awesome update today!
I took the bike for a spin two days ago with my jerry rigged ignition setup and it went well! I had two hiccups: One was some backfiring which seemed to have been caused by my ground wire slipping off, and the other was the bike stopping dead which was because ... I ran out of gas. In both cases it was a quick fix and a solid 30 minute ride went off without a hitch. The next day I went to re-integrate the magneto back into the wiring harness when disaster struck.

I got most of the way through fixing the wiring and I thought I'd try checking for spark, but when I tried it I got nothing. I reverted all of my work back to the exact setup as before and still nothing. I was already really frustrated by this, but I did find one clue: At certain positions of the flywheel, I would have a normal resistance value but for others I would have total discontinuity. I have no idea why anything could have randomly changed between me riding the bike and working on it except for maybe the extreme cold that rolled in that night or MAYBE me pulling a little too hard on the wires from the magneto, but in any case I figured the flywheel would have to come off. I took RT's advice and cranked that thing to hell and back so taking the nut off was tough, and I'm afraid that amidst my frustration and my homebrew puller being on its last legs I may have damaged the source coil. I was taking the puller off and I saw about one full layer of slashed windings. Nice work Daniel.

So now I have to wait around to get a real flywheel puller and lament my impatience before I get the thing off and try to figure out what could possibly have changed between the bike running perfectly, it sitting outside overnight just like it always has, me rolling it inside, working on the loom, and it not sparking at all. I did also connect a good battery and test if the signals worked, which they did, in case that's relevant (but the headlight did not. separate issue.) I also cleaned the spark plug which had tarnished somewhat but that shouldn't have been an issue since the bike ran fine when stopped.

Puller arrives Monday. Stay tuned for updates! Thanks again for the replies and (perhaps premature) congratulations. Also @uncle al, I do appreciate the input! No advice is too basic for me, I do greatly value it all. I did check the oil and it's looking good to me.

Look really closely at the wiring...  source coil to condenser and condenser to points.  Very, very easy to have a wire rub through insulation and sometimes very hard to see.  That usually shorts to ground though.  Not total loss of continuity.

One thing that hasn’t been mentioned is change your brake shoes front and rear very important 50 yr old brake shoes can and have become delaminated and you can go flying over your handlebars not pretty I personally have had brake shoes delaminate 

New semi-update. I bought a 27mm left handed thread flywheel puller per the recommendation of RT and it will not thread into the flywheel. I have verified both apparent thread directions and they match. I am unsure if the thread pitch is mismatched or if my flywheel's inside threads are just damaged due to socket use or something. I am guessing the latter, in which case it doesn't seem that there's any thread chasers available in 27mm left handed so I might just have at it with a pick and see what happens. More updates to follow. Thanks again.

The first time I pulled the flywheel on my DT400 I swear that the threads were messed up, I tried and tried and tried then finally it started to thread on. The threads are a fine thread so I had to catch them just right. Hope you're able to get it...

Most common puller out there 27x1 lefthand. Get a magnifying glass & look at the thread start. I have --in desperation--ground a groove length-ways my puller & tapered off the first couple of threads to sort of make it into a poor mans tap or thread chaser. Puller was a long beep version so i shortened it up after many years of use to get the best threads that were never used.
If you bought a puller that is too short can the crank bottom out before starting to thread in. Or is it trying to start but won't bite.
I looked back but can't see where i recommended a puller--if it was me, but have linked them in the past so hope i've not sent ya down the wrong path.
Also as i'm sure i've said--remove all washers to get the full puller thread used in the flywheel.

Found my puller--when i wasn't looking for it. So thought i'd take a pic. You can't see the end but i've shortened it years ago so groove i carved was likely twice as long but doesn't have to be. Looks probably due for replacement but still does the job. Pic coming-- i hope.
photos.google.com/search/_tra_/photo/AF1...EqnTZLzSq9HrD9SffUdD

Morley, got the dreaded 404 error when trying to view your photo.

Bugger. Hope this fixes it now i remember how the kids told me to go 'share'. Let's know please. & now i'd better see what others i posted links too today haha.
photos.app.goo.gl/3oFYE9kxDuYKyBLEA

That got it Morley, the other photos you tried to post recently in another thread is the same 404 error.