The PM has well over 100 miles of fishable water. I believe the flies only section has about 7 and Hal, in case you haven't heard, it is a no-kill section. As you mention, the upper branches and lower sections have plenty of spawning gravel for steelhead. The upper sections are closed Oct - end of April and the lower section has far less pressure than the flies only. There's plenty of good water if you know the river.
FWIW - The 2 canoe liveries each get 250 daily permits from the NFS and 100 are left for private individuals. Those are weekend numbers - they're less during the week. Permits are only needed from mid May to mid September and you can reserve them ahead. Boats are only allowed from 9AM-6PM which leaves good trout fishing before and after.
Guide Permits for the PM are another story. Permit numbers are prominently displayed on the boats of licensed guides. Not the same as watercraft numbers. There are only about 40 permits issued. Contrary to what some of you seem to think, there isn't a guide who's going to risk losing his permit by encroaching on other folks fishing and having someone complain to the NFS. The "guides" who might piss you off are likely unlicensed as well as unprofessional. These days, anyone who owns a driftboat seems to pass themselves off as a "guide". In addition to Guide Permits on the PM, all guides must also have a license to operate anywhere in Michigan. You'll learn from a good guide. Always hire one by someone else's recommendation. Email me if you're interested.
20-40K? Get real. Do the math.
As far as fishing gravel goes, why would a sportsman fish anywhere near bedding fish? Bedding fish make poor angling targets, day or night. Good fishermen leave gravel alone and lead others by example.