... the defenses mainly involve the taking of uke's balance and "plopping" them on their backsides.
Question. Is using the "lower cross hand block" really aikido anyway? Sounds like force against force to me.
Another question. What if the first kick is a fake? What I found worked very well was to fake a fairly low front kick, not too fast (or slow) to bring the arm(s) down, then to quickly change to a roundhouse kick to the side of the head, or even to a hook kick to the other side of the head. If you're fast, it usually works like a charm.