Weight loss advice? (low body fat but high BMI)

Don't worry about BMI.

I would go by how you look in the mirror and how you feel, both in your training and just walking around. If you were as fit as you've been at 9% I would not shoot for lower than that. single digit body fat is hard to maintain unless you are a pro and doing this full time.

What are your thoughts on muscle loss?  Sounds like your carrying around a fair amount of lean weight.  Great for the beach or for lifting heavy things but not very useful for things like marathon running.   If your doing any weight training stop.  Stop doing a lot of short hard efforts, sprints and such, and focus your training on long aerobic stuff.  Then just run a calorie deficit.  You will start to loose muscle mass and will start to gain that rib showing, joint bulging elite marathoner look!

No fat, no muscle makes for a fast marathoner!

If you know your actual BF, throw BMI out the window.

Calipers however, will not tell you your actual BF as accurately as a bodpod, a dexa, or as you mentioned, water displacement.

But the answer to your question, is the same as it is for a superclyde like me - eat less, move more. I don't mean to be a smart-aleck, but yeah, that's pretty much it.

If you're trying to fuel long runs in a high-carb way, check out the work of Dr Bob Seebohar on metabolic efficiency. I just got into the book, which is probably beyond my needs as just a regular old slow fat guy - but for someone with actual potential for marathon times, he may be talking right to you.

I go by this method. I have found that the longer you do this training the more you just get to know your body. I'm 5'6" (46 year old male) and race at around 140 lbs. I weigh myself once a week and I always guess what my weight is before I step on the scale. It is very rare that I'm not either right on or 1 lb off. I have no idea what my BMI is or what my body fat % is and don't care to know. All I care about is how I feel and how my race results are.