I think I would handle it like when you bring home a baby and have another child! That's how he feels and he's telling you he's not happy about it. I would pay him a little more attention, special treat etc. Maybe take him for a walk alone. Let the new corgi get on the couch and cuddle! They each have their own personality. So if he gets upset when you are cuddling on the couch talk to him and praise him. He is very cute! His high pitched bark is like "what about me, did you forget me, etc" My corgi always lets her feeling known. Hopefully in time, he will settle down with lots of TLC.
Some behavior that seems rough to humans is normal dog interaction. Each dog has to establish and accept its place in the dog pecking order, which may include humans in ways the humans don't fully grasp. (Just because you think you're the head dog in the household doesn't mean they think so; they may perceive you as more like a pup or a sheep, or just another rather strange-looking pack member.) Don't panic about barking and chasing around unless it truly appears one dog is about to be injured: think of it as a conversation... "I'm the boss here!" "Oh yeah? Says you!" This can go on for some time until a comfortable (to the dogs) dynamic is established.
You are entirely right to keep them separated while you're gone -- it won't harm either of them and absolutely will prevent any nasty surprises when you get home.
Ruby the Corgi Pup has a pretty dominant personality. I thought Cassie the Corgi was Queen of the Universe...hah! As nothing!
To make the transition of bringing Ruby into the household, I was always careful to put Cassie's food down first. For some time, I put Ruby's leash on her and looped the other end over the oven handle, so she couldn't barge over and shove Cassie away from her food. After a few months, she got in the habit of waiting until Cassie is done and walks away from her dish before running over to hoover up every single remaining molecule, so now does not have to be leashed for dinner.
They sleep on my bed and are too small to jump up there or off. From the git-go, I've always lifted Cassie up first and lifted her down first. Any other privileges are always accorded to Cassie first, then Ruby. This doesn't change the fact that Ruby believes she's the dominant pack member, but it does, I think, state that the human thinks she is not. They don't fight, although occasionally Cassie will lay down the law with a display of mock snapping and growling, none of which ever makes contact. Eventually you learn to recognize the difference between the "get out of my face, brat" snap/growl/yap and a serious confrontation.
Otherwise, neither dog goes on the furniture. Given the potential for conflict, I might discourage the furniture going, even though you do like to cuddle with the dog. Maybe get a stuffed dog? ;-)
I think i would try giving him some treats while he is on the floor and she is on the couch. It might help give him a positive feeling about her getting attention. Also, if it seems their play is getting a little rough i would call them both off and redirect their attention
First let me compliment you on taking in this girl whose owners have passed away. It does take some time for the resident dog to accept that the new one coming in is there to stay, but once they form their own bond, they will be both happier for it as dogs are pack animals and, much as we become part of their pack in pet situations, nothing really can fully replace the real thing ( another compatible dog). The transition needs to be managed by you, not by the dogs. You are the one who sets reasonable expectations and makes them stick. Pack order, in this case, means YOU are the one at the top and that needs to remain clear. The role of the owner is that of "benevolent King or Queen" the giver of all good things, yet still the one in charge, the one who sets the rules and sees that they are respected.
If any of the interaction you observe is not to your liking ( meaning not something you want to see more of ) curtail it immediately. You can do this by voice, if you have voice control, or by putting a dog on leash temporarily, while remaining in the environment. You need not be holding the leash, it can be attached to a sturdy piece of furniture, ready for a "time out" if needed, but in the same room where you are. Stop telling yourself he is guarding you, tell yourself he is claiming you as his property, something you are NOT! I would forgo too much cuddling of the new one, much as it is tempting, and keep the interaction more matter of fact until the two have integrated better. If she gets on the couch and you like her, fine, but don't make a fuss over her, keep it really low key and, if your boy starts his high pich bark, I would use the word "Quiet!" and expect him to shut up! The more excuses you make for the behavior you do not want, the longer it will take to turn it around as bad habits take root quikcly....
I also agree on not leaving them alone unsupervised for several weeks, or until they have become "old hat" to each other.
Nice walks together, weather allowing, are excellent. If you do things right, the whole process should take no more than two or three weeks. If you have someone else there with you, make sure all humans are on the same page. Best wishes for a peaceable kingdom.