I'm not sure I have this in the right section, the serial number of my new-to-me 18-3 is 5Kxxxx.
Feel free to move...

Two questions-

The front sideplate screw is tight, but the crane/cylinder flops in and out freely when the release is pushed forward and the cylinder is out of the window, unlike my newer guns that have a slight drag. It's almost too easy to accidentally "Bogart" the cylinder. Is this normal?

When cocked slowly SA, all 6 chambers will have the cylinder stop click into the notch. When doing this slowly DA, 2 of the chambers will not lock up before the hammer starts to fall, but will be locked at some point during/after hammer fall.
About once every 100 rds, the gun will spit something I can feel on my face, not major but noticeable. I haven't yet been able to determine which chamber(s) is doing this.
Flash gap is pretty even at about 4-5 thou, and end shake is too little for me to measure.

Suggestions?
Thanks

Could be the two round head screws have been removed then not put back in their proper places. If that's not it, replacing and fitting a new appropriate S&W screw is in order. Be SURE to use properly fitting screwdrivers.

Sounds like the spitting is caused by a carry up problem. A replacement long hand may solve the problem or not.

Best bet would be to find a good S&W pistolsmith and let them deal with the mentioned problems.

Are all K-frame screws identical?

I don't recall the details, but the screws on my new 19-9 looked different at the tip than others I've seen.

So, you didn't mention if the "spitting" occurred during single action or double action, but once every 100 rounds or so seems common to me for a .22 L.R. revolver.
As for the carry up in double action, unless you have an overtravel stop to keep the trigger from going past the break point, the sudden trigger movement after the break will roll the cylinder into the locked position before the hammer strikes, IF it's as close as you say it is. I have a few revolvers that do this. However, if the wear is more significant that that, it could be an issue.
The screw for the yoke retention should be the same on any older K frame (post war up to pre MIM parts era.) I may be wrong on this, but I haven't seen it.

The "flopping" could be caused by the yoke (crane) "button" on the end of the shaft is broken, by screwing the yoke retention screw into the button.

The occasional spitting has happened only during DA firing, which is 99% of my shooting with this gun.

The gun came with the factory trigger stop installed, I may remove it to see if anything changes. My 19-9 came from the factory with no trigger stop, and I recall that years ago LE agencies were recommended to remove any stops installed on their guns; because if the adjustment slipped then it could prevent the gun from firing.

I'll check that out!