Re: patio edging for offset pavers
Heave comes from below, so if you've set a proper base, which sounds like you have, then this won't be an issue. As far as shifting, that really isn't going to be a problem unless the step stones are on a pretty good incline, which doesn't like the case.
When you remove the wood spacers and install the concrete, allow the concrete to be 1" to 2" deeper than the step stones, this will act as cleats and hold things in place better. As the succulents and grass grows up to the edges, their root structures will also stabilize the soil and solidify the edges.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!