>Basically that's no more than using force at a safe distance.
No, if you read what I wrote above, I said 'wiggle the long line' - purely to get the puppy's attention. That is not 'force' - I'm not pulling the puppy anywhere or making it do anything.
>so I just wanted to know how best to go about moving her without escalating the situation or making it worse in the future.
St Domingo, I'm an APDT dog trainer who teaches many different classes, offers one-to-ones and has worked with hundreds of puppies over many years. I specialise in early development and have several puppy classes running at once.
I strongly advise that you use a houseline if you need to get the puppy's attention.
>I don't think she would move for a treat as I think she would prefer the prized comfy spot to a treat,
I would encourage you to consider what your idea of a 'treat' is, if you think your puppy would prefer to stay there rather than have a 'treat'. A treat which is going to work in this instance isn't a Bonio or a processed dog treat, it is something incredibly tasty like a bit of ham, chicken, hotdog - essentially, just go to the fridge and see what you have left in there which you could use. I can assure you that if treats can obtain reliable recalls out and about with great distractions, then treats can also encourage a dog to move off the sofa. It's just a matter of finding a treat which is high enough value to be effective.
>and also I am concerned about rewarding the bad behavior of growling.
This is a concern which owners often have. Having worked with many resource-guarding puppies, it just doesn't work that way. The reward will most strongly reinforce the behaviour which just occurred. And the behaviour which just occurred was getting off the sofa: SO your treat is going to reward getting off the sofa, not growling.
A further reason why the growling won't be rewarded, is because you are not going to wait for a growl any longer. Knowing that your puppy has this propensity, you are going to anticipate it and - whenever you need to move the puppy, for now - go and get a treat, in advance of any growl. In short: There will be no growling happening, to be rewarded.
>She has already worked out that if she grabs something she shouldn't have, such as a dirty sock from the washing, that she will get a game of me chasing her then a treat to swap when she finally decides to swap.
This is the beginning of resource guarding. Given that your puppy is already growling when you want to move her, are you now going to chase her when a 'hot' object is in her possession, and then force her to give it up to you? If she 'has already worked out that if she grabs something...she will get a game' - who has taught her that? You! If you show any desire for the object she has, or make any attempt to get it, you are communicating to her 'that object is highly desirable'. She is only going to want it even more, if she sees that you want it.
The solution is to pretend total lack of interest in the object she has taken, go to the kitchen, get a treat. (By that, I mean - something incredibly tasty, perhaps even a blob of pate, garlic sausage - whatever works.) Put the treat on the puppy's nose and STILL make no attempt to take the object. When the puppy smells the treat, she will release the object - STILL do not take it - because she will see you. Instead, lure her forwards a few steps so she passes the dropped object, and, when her back is turned and she's not looking, then pick up the object. It's a bit like a magician's sleight of hand. The most important bit is that the puppy doesn't see you pick the object up. For some reason, if they turn around and see it in your possession, it has no effect - what you don't want them to see is you taking possession of it. (This is also the basis for the clicker retrieve, when you have a dog which has a tendency to value objects and teaching the clicker retrieve can also be an extremely effective solution to resource-guarding of objects. But that is a bit more complicated, so I won't get into it.)
It takes many words to explain this, it takes only a few seconds to do it. It pretends resource guarding before it has even happened and it should be taught in every puppy class. You can start to put a word to this (Leave), and eventually you'll have a dog which will immediately leave things it has picked up, because it believes a treat is coming. You can then sometimes provide a treat and sometimes not.
If you continue to chase the puppy around - and this is a puppy we already know has guarding tendencies - you are really pushing things towards the puppy beginning to guard objects from you as well, and seeing you as a threat to the object they've taken possession of. Setting yourself up in opposition like this really doesn't work. I have had people come to me with young dogs which have bitten them, as a direct result of them chasing the dog around the house. Only last month a tibetan terrier owner told me they were considering rehoming their dog as it had bitten the cleaner and their 17 year old son, after running off with a feather duster. To them, this had happened out of the blue. When questioned what had happened in the past when the dog had picked something up, it emerged there was a long history of the dog running off with things and being chased and forced to give them up. Even the dog's growls had been ignored because these people didn't believe their dog would actually bite them. When this behaviour first started, they thought it was kind of cute in a puppy-ish way.
I can warn you repeatedly and I can point out what is likely to ultimately happen. I can't 'make' you do what I suggest. Perhaps you might at least try it.