This is my 72 W200, 318. Converted to electronic ignition and using the blue HiRev 7500 box.

Today, since it was cold and had been a while since I started it, it cranked slow... and never started. I pulled the coil wire off and cranked it, and had no sparks, except sometimes when I released the key.

Put it back together and put the battery charger on it. Few minutes later it cranked slightly faster and fired right up.

Later when it was warm, I again removed the coil wire and cranked it over. Now being warm, it cranked MUCH faster. I had a short (~1s, 5-8 revolution?) delay between when it started cranking and I got any sparks, and even then had the occasional missed spark.

I was thinking that I should have a good spark immediately. Am I wrong? Any idea why I wouldn't have it immediately, or when it's cranking slowly?

I'm already looking at a new mini starter to fix the slow cranking part , but the delay on the spark has me

that is a strange one. since the reluctor rotation is what gets the ball rolling I would start there & (1) check it for (correct) gap. (2) clean all connections/plug ins including the bulkhead (#1 place) (3) I wouldn't suggest shotgunning new parts at it but if you have replacements in your stash you might sub in another ECU and a pickup (whole dist might be easier) one at a time. You have no choice as (likely) it's gonna get worse & will eventually leave you stranded (usually at the most inopportune time) & it wont fix itself. Holler how it turns out. Myself I'm thinking ECU or pickup

make sure your ECU has a good ground...you may have to scratch a little paint off the new ECU.

As I think about it more, I do recall the reluctor slipping farther open on me a couple years ago

Time to take the cap off and check it.

I suppose I could also pull the distributor, then manually spin it and watch the (non) spark.

plug in a spare one (even a /6) & spin it. it'd be easier to elim/confirm the dist that way if you have one handy.

Took the distributor cap off tonight, and checked the gap between the reluctor and pickup. 0.012".

It's supposed to be closer than that, right? 6-8 thou I thought. Will another 4 make that much difference?

To answer my own question; yes, it is enough to make a difference.

Pulled the dist before making any changes and spun it to watch the sparks. Fast spin = lots of sparks. Slow spin = sometimes spark

Closed the gap to 'I can still see light' on the close side, and ~10 on the other side. Similar with the vac advance engaged.
Now, fast spin = lots of sparks, slow spin = many more sparks than before.

Then it cranked and fired right up much faster than it normally would have.