I have this problem with two of my cockers too. Both have had the op and one has had it done twice and still suffers.
There is a cream you can get from your vet [they may have to order it in] It's called Flamazine and is very good. I was given this from a specialist vet, use twice a day. I've stopped using the hibiscrub and clean with eye wipes. [figured they'd be gentle] Dry the mouth with kitchen roll after every meal and drink. If you feed kibble try wetting it before feeding, I have found this has helped Paisley.
The dryer you can keep around the mouth the better.