I'm sorry but your post appears to be one contradiction after another. Could you please answer the following questions so we can help you help yourself :
has black sooty water collecting in the bilge.
What evidence do you have this is exhaust water ? Pictures please.
I also have clean exhaust water collecting under the engine.
What evidence do you have this is exhaust water as opposed to lake cooling water ?
As far as I can tell the exhaust elbow is not cracked.
What have you done to determine if it's leaking or not. If I checked it during operation, it is either leaking or it's NOT leaking. It's not "as far as I can tell" it's not leaking. Pull the elbow and inspect it AFTER you have wrapped it in paper towel. NO, not on the extremely hot riser as there's no water there. Onhte cooled portion.
I’ve seen posts where the exhaust muffler and fuel tank had some kind of incestuous relationship and exhaust water was getting into the fuel tank.
And I've heard that some guy in Kalamazoo has built a perpetual motion machine.
This is crap from the internet and you should be able to determine by your own inspection that there is no way exhaust water could to get into your fuel tank. Unless the engine exhaust line is connected to the fuel tank vent line ? ? ? ? Maybe ?
Also saw a post where the partition between the engine compartment and the starboard lazarette was removed to facilitate access to exhaust elbow.
Are you saying you are having trouble accessing your exhaust mixing elbow ? Is this the reason you are unable to determine if the elbow is leaking ? Pictures and an explanation may go far to helping you with this problem.
do I need to replace stock muffler?
Is there anything wrong with the existing muffler ? If it's leaking, by all means replace it as it's a source of carbon monoxide. If it's based on this idea that the exhaust system is somehow putting exhaust gasses and water into the fuel tank, maybe not the best idea I've ever heard. Maybe save your money for something more useful like investing in the guy with the perpetual motion machine.