You’ve peeled off too much information. In this example you only need to track the element (in the form of the specific ElementRegion), but you’re trying to track the whole array instead. The information about index is crucial for that. You should not peel it off, but instead use it.
Also, the trackNullOrUndefValue visitor is a mess. These getDerefExpr/peelOffOuterExpr/etc functions have really weird contracts that nobody understands. Getting anything done here is very hard unless somebody actually does the work of decomposing the solution into simple clear steps (which is, well, the hardest part of programming).
If you ever need to see if a LazyCompoundVal is fully or partially (un)initialized, you can iterBindings(). This is non-typechecked lookup (i.e., you cannot figure out which specific sub-region is initialized by the value you look at, you only get a byte offset). You can also improve iterBindings() to make it possible to restrict the iteration to a single cluster, because within the LazyCompoundVal’s Store only the cluster that corresponds to the LazyCompoundVal’s parent region is relevant.