To my relatively untrained eye, it now appears that the local red-tailed hawks are no longer interested in the nestbox, since we've removed the perch. They're still around, although from my observations, they're just using the nestbox hill as one of many local hills that are available for ridge soaring and hunting. I'm reaching the conclusion that they are pair bonding and probably getting ready to build a nest, but it doesn't appear that it's nearby (I've scouted out the area immediately around our house and hillside, and found nothing). One of the pair, presumably the male, was observed yesterday flying around with a couple of sticks in his talons. My understanding is that this is not necessarily nest-building per se, but preparatory activity as their hormones start to kick in with the lengthening days.
We often see 2, 3 or even 4 red-tails (the local pair perhaps included) hunting and hovering above a very large hill a bit further away to the northeast of us, and several are often seen in and around the greenbelt area and park to the south of us. I don't know what the average red-tail territory is here in SoCal, but it must be quite large.
I still think that there shouldn't be an issue between any red-tails around here and any kestrels, as the two species have lived in this area near each other for many years now (indeed, perhaps for thousands of years), sharing territory and occupying different ecological niches. As long as the RTHs don't get it in their heads to try and eat any kestrels, we should be okay. Keep the good vibes coming for a pair of kestrels this year...