So, I have never worked on a SPT, so can take this with a grain of salt . . .
But on the SPT contract, there are no mention of previews. Under definitions, there is no preview definition . . . given that, the do tell you what "opening" is.
Opening. "Opening" shall be defined as the first paid public performance.
So, although a theater may define a performance a preview for marketing purposes, I read the contract that your first paid public performance would be opening - and thus all performances, marketed a preview or not, would count towards your performance count.
Do check with AEA, but the way I read it . . . and the fact the word "preview" doesn't appear ANYWHERE in the contract, it lends weight to the fact a performance is a performance.
Now, I also disagree with the fact if they do a non-paid public performance . . . if that would count as a performance a not - if it's public, it should count as a performance regardless if people pay or not.