You've opened it all up to make sure there's a valve that's sticking? Pull it back apart and measure the spring and check the shim bucket for any signs of pitting or other abrasion... Some sludged up oil shouldn't cause a valve to stick, not by itself anyway; the shit simply isn't thick enough to hold it unless the stem or the bucket is damaged and providing a place for debris to get wedged in.
I am not a fan of seafoam for crankcase cleaning. The stuff is not much more than mineral spirits and naptha... naptha is one of a few chemicals which dissolves silicone, not very desirable. There are other products out there which will do as good of a job but are friendly to the oil and seals... the downside is that you have to be patient.
A good oil will have these detergents in them already, but if you feel more powerful cleaning is needed look for an ester based detergent such as AutoRX. Detergents are the preferable way to go, IMO, as you can actually leave them in the engine to do their job. They don't go in for a couple hours, penetrate a millimeter or two into the sludge and then get drained before they could do more due to the dangers they pose to the rest of the system.