On your lay bin I would not do 8+ inches you don’t want them to dig unnecessarily they usually go to the bottom of the bin and to dig so deep Wastes so much valuable energy some people put a couple feet of soil and the thing is so exhausted Unnecessarily I say 6 inches is fine
if there is NO obvious infection ,sterile SALINE 1OO percent saline drips to flush a speck of powder or shed or ? does wonders! if after a few attempts no luck then a vet