I don't know how much practice he is really getting. He goes through 5-10 topics per hour, based on difficulty and attention to detail at the time, even including drink and potty breaks. This has caused two areas of concern.
5 topics an hour wouldn't worry me unduly, but if he's doing 10 topics an hour regularly, I suggest that he already knows the material and is working on things that are too easy. At least in the courses I'm remembering from recently, it usually takes 3 questions correct (with none wrong) to get a topic on your pie; if he's doing 30 questions in an hour that also includes the odd break, he's obviously not having to think about the questions much.
Second, every session ALEKS queues up a new interim assessment for him, and I have to call and get it cancelled
Huh? Why? I've always just let DS do the assessments the system schedules, even if it's frequently, because they aim to fix exactly the problems you're talking about, and IME do it pretty well. (By the way the underlying "knowledge space" theory is cool, too.) Because the assessment not only asks you questions on what you've already done (at most one question per topic, if you get it right) but also asks you questions on things that are not on your pie yet, answering on an assessment something you haven't officially done yet is the way to get over the boring stuff that's obvious if you really understand what you've done so far and to the genuinely new stuff. (Especially in the earlier courses, where the increments were often tiny, it often happened that the system would ask DS a question on a topic that wasn't on his pie and he wouldn't even notice, he'd answer it based on what he'd understood. He could skip whole prerequisite chains of topics that weren't necessary for him, that way. That's fine, because those are exactly the topics I don't want him to have to get one at a time by answering three questions on each.) Similarly, that's where you find out whether more practice is needed - if he can't answer the question, the topic'll get taken off his pie and he'll get more practice. Does your DS object to doing assessments frequently? Why? Is it possible that this is fixable?
It doesn't seem ideal as skills practice for a highly gifted kid because of lack of customizability coupled with assumptions about learning at a normal pace;
I'd love to have the full teacher interface available to select additional topics and maybe leave out the odd one (synthetic division! crazy thing! although I have to admit DS enjoyed it), and there'd be no harm in being able to fiddle when it assesses, although I don't think I'd use it even if it were there. What other customization would you want, just out of curiosity? (For context, my DS is still certainly not "learning at a normal pace" - in our most recent subscription episode, he was signed up for 4 months, during which he worked on it only at weekends and a little during the holiday, was logged in for I think 50-odd hours total, and in that time did both Geometry and Algebra 2. However, it was pretty rare for him to work for more than an hour at a stretch in it, and also only in the holidays did he ever do more than 2 consecutive days, so I suppose we didn't encounter the really frequent assessment issue that you may have; he usually got a week at least between them.)
but it also doesn't seem ideal as a way to actually learn or teach new math concepts, because the bare-bones explanations don't qualify as lessons, though I am in favor of letting kids noodle through things on their own a good part of the time.
For learning I like it a lot (I think a set of gradually harder problems such that in fact you can often work out how to do the problem without any actual teaching is great), but for teaching I agree it's no use. We basically never use the explanations in ALEKS; if DS has trouble he asks me or DH.
