Questions:

Do I mix stucco myself, or buy premix? Do I need a plain layer of
lath over the block first? Or a layer of wet uncolored scratch coat
stucco and a layer of cloth imbedded, then let dry, then a topcoat of
colored? I want something that will last the longest without popping
off in chunks due to our temperature swings from 15 F to 115 F, and
the wet/freeze/dry cycles of rain/ snow, baking sun.

Steve, I'm looking at the same questions you are somewhat. What is
your location? (If it's AZ, don't worry, I'm not coming over to
celebrate the enactment of civil rights with y'all.)

I don't think summers are going to be getting cooler in the southwest,
so I'm hoping to be the pig that wolf or sun can't do in.

Cheers,

The blocks will have an invisible coat of bond breaker that made it easy
to get them out of the molds. That's a very light oil that will prevent a
good bond of the stucco onto the blocks if you don't clean it very well.
I use TSP because it doesn't leave a residue - make sure you rinse well
after. All of your suggestions are good and will work. A very good
modification is to use acrylic admix instead of water or at least a 50/50
mix of water and admix. The admix improves the stickiness and helps with
freeze cycles. I did two 8' x 10' planters- clean like crazy, rinse like
crazy, damp the blocks but not wet (no free water at all), one coat
colored stucco and one coat top coat. Cloth and lath is not required and
will cause looseness later if the cloth is not totally saturated which is
almost impossible. So leave it out. Any voids would be places where water
will enter and freeze and expand and loosen the stucco.