So is this right?

Just if that much variance in the cam index marks might mean a slipped tooth, and possibly an explanation for my valve situation.

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It do.

Keep in mind that cams rotate 1/2 speed of crank so 4° - 6° at the cam translates to twice that at the crank. And that is fairly significant in terms of cam timing events (which BTW are measured in crank degrees)

I had a pass side cam skip a tooth once (retard cam timing on that side) and let me tell you - it can't possibly go unnoticed! The motor ran like one bank of injectors was squirting molasses instead of fuel! Got lucky that day. But as soon as it happened I took it easy and drove her directly over to Giuseppe.

But, as mentioned. It can survive that. Judging by the fact that the white belt-to-cam marks the PO put on there are aligned. Your motor was set-up like this. If you got a little rev happy 'troubleshooting' why the motor was not purring like a kitten, that could have been how that valve got formerly introduced to it's neighbor downstairs.

The distributor drive pulley orientation itself is not as important as the distributor rotor orientation as the cap only goes on one way and the rotor must be under #1. Marking it can be helpful when installing a new belt as it can be easy to rotate it a few teeth by accident if it's a tight fit. Not so much a problem with a mechanical de-tensioner as with a hydraulic unit.

I did nudge the pulley backward just a smidge, since I was looking at the mark on the 3-6 cam and I'd overshot the mark on the 1-3 cam, and VERY carefully/slowly, then cranked it clockwise again just to pull out any slack that might have been created. That's how I know there's a misalignment. I'll fix it when I pull the belt and the 1-3 head, which is what I'm actually in the process of doing.

So does a new timing belt come with the paint marks already on it, or is that something you do once it's installed? As of now, the marks are all lining up, even though the cam index marks don't. Just assuming the PO installed the belt one tooth off, then painted on the reference marks (incorrectly placed).

So the distributor is just off a full rotation? I assume the rotor turns 360 for every half turn of the pulley? Right now it's on #1, right at the mark. I guess I'll see! Like I said, everything was firing in the correct order as it was, so in reality there's not an especially compelling reason to adjust it, assuming someone in the past has already compensated?

Yep, that is a mechanical tensioner - don't rotate the engine backwards! In regards to the white marks - whenever I change the cambelt I paint liquid paper marks on each pulley and the belt. Put the same marks on the new belt and then when I put the new belt on I'm certain to have everything lined up properly - of course if the cam already skipped a tooth this isn't going to work and the new belt will be in the same wrong position. This may explain why the PO's white marks are in the wrong spot and also why there might be white marks on the belt and pulleys, cause that cam timing is definitely wrong.

BTW once you rotate the engine the white marks on the cam belt and pulleys won't line up again for many, many revolutions. That may explain why the white mark on the distributor pulley is 180 out - the scribed marks on the cams and distributor case and TDC are what you need to use to check alignment.

This is why when acquiring Alfa's
1) Never buy one "that was running good when it was parked".
2) If purchased already and no proof of when belt was last replaced (running or not), Replace the timing belt. In doing so, you can make sure the mechanical timing is correct.

Anyhoo, since you're learning now that it makes no sense (and ruins good sleep) to go off half ****ed with theories that have no basis in fact - like dangling valve seats, ECU leaning out et al.

So I guess I will leave you with a little something rare. Found this on the innernet a couple of weeks ago by pure luck (attached) Shankle instructions on replacing cams. A good addition to a shop manual. Which I would advise you get as well. A Milano Shop Manual will do for the job at hand and much, much easier to find. Might even be one floating around in one of these threads if you search smartly.