Cavaliers are shadows -- they have hundreds of years of breeding in their history to be this attached to their owners. It is one of the points I highlight in '
considering a cavalier' -- which I always ask prospective homes looking for my rescue dogs to read. This behaviour is a hallmark of the breed and for some it is one of the things we love about cavaliers. For others it may be the main reason they decide to give their dog to rescue. Depending on the point of view it is either a positive or a negative but it IS the norm for the breed.
Cavaliers are the wrong breed for someone who wants a dog that will sit at a distance and watch what goes on. That said, younger dogs and puppies tend to be a bit more shadow-like than older dogs who know the pattern of what happens each day. And close cmpanionship does NOT mean allowing a dog to be pushy and rude.
If you want him to give you space then you need to get an xpen or similar and train him to accept being put inside and spend some time on his own. You can also crate him, for short periods or confine him to a room (with a stuffed kong, water, a bed etc). It is a good idea to have a dog trained to be on its own. There is info on this under separation anxiety in the Library section. He is also well into the time when you should be able to have a dog trained to go into a downstay or to go to his bed to let you do things without him sitting on top of you. If you haven;t taken him to a training class I'd advise doing that immediately. If you have gone to a class, it sounds like some daily review and practice is needed to get him to do what you'd like.
If he is barking in an unwanted way when your partner comes home then there are a few things you can do -- for example, give him a time out -- no comment, just lead him to a room and confine him for 5 minutes, then let him out again without comment and totally ignoring him. You should also train him to the command 'shush' or 'quiet' (which usually means training 'speak' first).
If you don't have a puppy guide I strongly advise ordering Dr Ian Dunbar's
Before and After you get your Puppy, from Amazon. It isn't a good idea just to wing it as the prime time to train is now, while he is young. It is much easier to train to get the dog you want then to try and address problems that may become long term issues if left until adulthood. As you rightly note, what is funny or cute in a puppy is not going to be funny in a loud and pushy adult.