Heaving is usually only an issue if you're building something structural - something that needs a foundation or is stacked more than just one or two layers high.

I wouldn't worry about it. Essentially what you have is edging. A little heaving won't hurt it or even be noticeable in most cases. It's easy enough to dig around a block or two and reset them if you see something noticeable.

(not exactly commenting directly) It looks to me as the blocks don't help as much as they could. They form a raised bed (good especially if you need better soil or better drainage), but they do not form any edging so when you mow you probably will need to weed wack. if you do use blocks like that, put a course of bricks down (not as a base - the base is unneeded) so that the mower wheel can ride on and save the extra step of trimming (or just edge the bed with a spade and have no blocks. I have become less fond of blocks and even flat bricks that the wheel can roll on altogether, versus only using the spade, The blocks seem more cost and more work and just edging lets me expand a bed anytime I want. Note: I have no aesthetic sense.

Not sure how this relates to square foot gardening. If you're not building up layers, there's no need to trench. If you're going to go up several layers, the bottom layer should go in sand below the level of the soil.