Hi! Jean Donaldson‘s book „ Mine“, might help you with the resource guarding. Using treats to lure him off the sofa is a good idea to get him to understand the command - off, down or whatever you use but I found just using it as a bribe had the opposite effect, -:they‘re very quick to make the wrong connection and use it to bribe you instead! (If Humphrey decides he wants a biscuit, he‘ll ask to go out ten times in an evening in the hope he‘ll get a biscuit for being good and coming back again pronto!!
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Tapping the nose is what I was told to do 40 years ago, along with biting their ears to stop snapping and all the other outdated nonsense - I‘m at permanent loggerheads with a neighbour who insists on punishing his dogs by causing them pain and it doesn‘t work! Cockers, especially, are very sensitive and respond much better to positive methods, far better to teach an alternative behaviour which can be praised than scolding for behaviour you don‘t want , (if that makes sense?)
Don‘t know if you‘re familiar with Zak George? He does a lot of Youtube dog training videos and only uses positive methods, - might be worth having look, I like his style, he uses lots of dogs and lots of breeds instead of pretending to be training a dog that is obviously quite perfect to start with!