UPDATE -

I went after the crumbles with a wire brush.

Most of the crumbling was old paint & some patch cement that had been
slapped on and decayed.

Only a few parts were of much concern. In these places, the mortar had
gotten very soft. It crushed like chalk. I think there was some bad
mortar, or it decayed over time. Using a masonry bit, I chewed away
the soft mortar. Then I enlarged the hole as prep for some hydraulic
cement patch.

For the worst hole, I drilled through to the block cavity and filled
it with expanding foam. Then I patched with hydraulic cement after the
foam cured.

Overall it's not too bad, really. It wasn't widespread. The blocks
were intact. It was only the mortar that was decaying. We'll see how
it all turns out.