Question #1: Is 10 “of wood chips sufficient over gravel?
Might do the trick, but they can dig a pretty deep hole.
If you hadn't already covered the HC run 'floor' with gravel, I'd suggest you remove it.
I think around 3 inches thick of wood chips would work quite well. You would rarely if ever have to clean it out as it would cold compost in place. Just add more wood chips on top periodically. I believe @aart uses wood chips. Perhaps they'll weigh in.
Yep 3-4" will take care of the poops, but not with HC underneath.
#2 I am debating on sand verses shavings for coop, which is raised and has a plywood floor
Not a sand fan, unless it's on a poop board.
What kind of bedding you use may depend on how you manage the manure.
This is about cleaning, but covers my big picture:
-I use poop boards under roosts with thin(<1/2") layer of sand/PDZ mix, sifted daily(takes 5-10mins) into bucket going to friends compost.
-Scrape big or wet poops off roost and ramps as needed.
-Pine shavings on coop floor, add some occasionally, totally changed out once or twice a year, old shavings added to run.
- My runs have semi-deep litter(cold composting), never clean anything out, just add smaller dry materials on occasion, add larger wood chippings as needed.
Aged
ramial wood chippings are best IMO.
-Nests are bedded with straw, add some occasionally, change out if needed(broken egg).
There is no odor, unless a fresh cecal has been dropped and when I open the bucket to add more poop.
That's how I keep it 'clean', have not found any reason to clean 'deeper' in 5 years.