Progress report on my ice cubes. Thanks for all the feedback. A few comments deserve mention:
1. VE1BLL suggested raising the freezer. I don't understand that one, so haven't tried it.
2. The quality of the ice wasn't in question, so I haven't tried distilled water.
3. Not getting involved in the whisky/whiskey fundamentalism...that's been done a lot.
4. We did have an icemaker fridge at one time, but after it almost burnt the house down, I don't want another.
I think the answer is related to the other suggestions, but will just tell you what I found in my simple tests, in which I froze cubes with the trays in four different orientations. Trays sitting on glass shelves in freezer compartment of side-by-side fridge.
1. Both trays filled, bottom tray cubes stick and crack, top tray cubes pop out perfectly.
2. Bottom tray filled, top tray empty, cubes in bottom tray stick and crack.
3. Bottom tray empty, top tray filled, cubes in top tray pop out perfectly.
4. Both trays (or one tray) filled, but not stacked, cubes pop out almost perfectly.
So it looks to me like the bottom tray cubes stick and crack due to being covered, while the uncovered trays produce good cubes. So it looks like unimpeded access by air to the top is the key. I will leave the technical reasons to you smart guys. It would be interesting to see whether someone else can repeat my observations.