Oh dear, this is turning into a serious issue for you, but you are moving on it at just the right time, when she is young and easy to train away from this.
I hope the trainer was not suggesting using punishment based approaches but instead, training your puppy -- and ALL puppies need such training as early as possible! -- to allow for food to be taken away and swapped for something else of equally high value, then swapped back, etc. But this shouldn't be done with the other dogs there! That's like putting her into an already tense situation and adding extra unexpected stress where she's supposed to be OK with worrying not just about the dogs but about you taking her food. And I'd not even work with her food bowl immediately, but with treats between meals.
But let's start from the beginning. If you have more than one dog, it really is never a good idea to feed them all together, in reach of each other. That goes for ALL dogs, and it is nothing to do with age, number of additional dogs, or gender, though the more dogs you add, the more likely problems are going to occur of all sorts as whole relationships and habits between dogs have to be rearranged.
Cavaliers are no different from any other breed, large or small, and I have seen cavaliers with lots of resource guarding problems ranging from mild to dangerous. The outside package may look cute but they are dogs, with every potential dog issue.
So: you are at the point where responsible management is need on some things, and on others, training. No amount of training will prevent dogs potentially fighting over food. This is a good opportunity to feed in crates. Or in separate rooms.Or a pen for one. Or keep your existing two well apart in one room and the pup in another room. Never feed them -- especially her -- where other dogs' food could be a temptation. Good trainers know that any dog, any age, can become food aggressive in this type of situation. Given what you are experiencing, it is also a good idea never to offer high value treats to all dogs out in the open. Put them in crates to eat them, or put her in a puppy pen; whatever. You do NOT want aggressive dog fights.
I have two half brothers who adore each other BUT in certain situations will guard high value food related items and this can end up in very alarming fights (and I have been bitten breaking them up). So it doesn't matter how nice the dogs are normally. Some are more prone to guarding a treasured item and the way to manage this best is not to give them the opportunity to end up in a fight.
Sitting between them sounds, frankly, pretty useless as a suggestion. Life is too short to have to sit amongst eating dogs every mealtime! Separating them by barriers of some sort -- your choice -- is the right approach.
Guarding on the couch is a different issue but also dealt with in a positive way. Rather than grabbing and moving a sleeping dog, instead train her to a cue to get off. Don't use 'down' if you also use down to mean lie down. Use 'off' or whatever you want. But lure her off with a little bit of cheese or valued treat and then praise. Eeventually you link the spoken command to the action.
You can learn how to do this if you download Dr Ian Dunbar's free book After You Get Your Puppy on
www.dogstardaily.com. If you register (free) you can do this and see other video and written resources on all these issues.
At 6 months your puppy is definitely ripe to get into a good positive approach class. I am not sure where you are based but I'd look for one with an APDT or CPDT or Stilwell Positively certified trainer. Too many clueless people are out there who will advise managing dogs in ways that make many problems worse.
If you check the Training section of the site there are many training site resources pinned to the top of the section and I'd also recommend reading the 'things every owner should read' post, which has tips for managing problems to resolve them more easily.
