ASTFactory, LeafPsiElement, ASTWrapperPsiElement and CompositElement

In the Custom Language Documentation, it is suggested that plugins should use ASTWrapperPsiElement as base for their PsiElements and I followed this rule. What always bothered me a bit was that my PsiTree contained one additional leaf element that just wrapped the underlying token. For the simple example of "f[a+b]" my tree, therefore, looked like this

So while my "Symbol" PsiElement should have been the leaf element, it was always a wrapper for an additional PsiElement that was created by the default ASTFactory. This led to situations where things like "findElementAtCaret" returned the PsiElement(IDENTIFIER) and I had to compare IElementTypes to find out if it is a variable. Tonight, I finally took the time to tackle this. I implemented an ASTFactory and changed my "atomic" PsiElements to LeafPsiElements because I didn't understand why leaves shouldn't be used as normal PsiElements. Now, my PsiTree looks really nice and clean:

It didn't take long to realise why this was a bad idea: When I now call getChildren on my FunctionCall element, I only get PsiElements that are composite because this is how it is implemented in the underlying method of ASTDelegatePsiElement

if (psiChild.getNode() instanceof CompositeElement) {
  result.add(psiChild);
}

Therefore, do I understand this correct that

1. I can use my ASTFactory to create customized leaf elements that I can test with instanceof instead of comparing IElementTypes?
2. I should not make my Symbol PsiElements the leaves of the tree and rather keep them as parent of the underlying token-PsiElements?