This thread really does prove to me that problems like this will not be solved on a forum, as you will get different opinions based on some sound theory and others based on personal experience. The truth is every dog is different, owners do things differently from an early age which can make a difference.
Yes i can take anything from my dogs because they trust me 100% and I know how they will react, but I certainly would not go into a clients house not knowing the dog who was growling and guarding and expect to be able to take the item back.
If people are going to give advice it has to be advice that is not going to get folks bitten, personal experience can not solve the issues of others as they are not in the Op's situation.
I really don't give out that much behavoural advice on any forum these days as I do not deem it appropriate without first seeing and assessing the dog.
I as in all cases such as this these days would advise the op, if she is worried to employ the services of a suitably qualified and experienced behavioral trainer to assess the dog and make appropriate suggestions.
In the mean time I recommend the problem is managed by denying access to the guarded items in the first place and under NO circumstances should they front the dog up and confront it.
With regards to tuggy, i have 3 cockers and a field spaniel who all play tuggy with a specific toy but when retrieving have perfectly soft mouths.
I guess if you work your dog on shoots then it is not something you would risk doing, but for the average working cocker living as a pet then i do not see any harm in such a game, OMG ,listen to me now, i seem to be speaking from experience.