He sounds a lot like my Yuki (not a corgi, but an American Eskimo dog) regarding the barking, to be honest.  He will bark at leaves rustling in the driveway, people walking by (talking - our windows are too high for him to see out), dogs barking, and heaven forbid someone knock on our door or ring the doorbell.  He takes his job as guardian/alarm very seriously and it didn't start until we moved into our current house from our last rental, which was also shortly after he turned 2 years old.

Yuki has been to obedience (after this new behavior), we've tried a no-bark spray collar, we've tried training a quiet command.  Nothing has worked so far and I've simply come to terms with the fact that this is just the way he is.  He's just a somewhat nervous/anxious boy who is relentless with his alarm bark unless I yield him and get him to come down from a level 8-10 alert level.  So it's possible that Einstein has nominated himself to be the house protector!  :)

Becca has gone through spells where she is on hyper alert. There was construction nearby for most of the summer. She felt the need to keep watch. I left the windows down on that side of the house and had background noise on, tv, ac, or white noise machine. The background noise helped.

Becca also guards food, but only from the cat. I feed her in her crate and the cat gets fed in a gated room. She seems to be more secure in her crate.

Have you looked into natural stress remedies? I have used the Vermont Naturals chews for my cat. I always keep a bag on hand.

To answer your original question (is it this or is it that?) I would say that it's all of the above.  Each one of the changes you mentioned would require a major adjustment from the dog and he kind of got them all too close together for him to process and integrate in a way that matches your expectations.  Since you can't go back and space any of the changes out, try a combination of avoiding the stressors you can and sticking to your guns on the important ones you cannot overlook, such as nuisance barking.  This may require further changes, for instance in feeding patterns.

I agree with those who say you should not leave out his food.  Feed him while you are still home, then pick up the bowl if he does not eat it.  I would not even wait 15 minutes.  A hungry dog will eat immediately, maybe fast or slow, but not stop eating.  It may take him a few days to figure it out, but he will get hungry and eat at the two designated times.  If he's not eaten his morning meal, you can give a bit of food when you come home for lunch to check on the puppy, but the idea is to have him hungry at dinner, where again you will not leave food just laying around at his convenience.

A new house with wood floors, especially if sparsely furnished, with not much in the way of wall hangings and curtains/fabric to absorb sound can really accentuate every noise and anything can set him off that he's not used to.  Teach him "quiet" and enforce it without making excuses.  Make sure you leave some soothing background noise in the house when you leave.   Limit his time outside alone as much as possible, bringing him inside at the first woof that follows your "quiet!" command.  If you are out with him and he ignores the command, put him immediately on leash and remain outside to enforce your rule.   It will be "one step forward and two steps back" any time he gets away with barking, so don't let up on that.

He may also have become more dominant as part of his maturing and the new pack order.  Make sure he knows you are the benevolent Queen, and he is in no way the King :-D

All this will take some time.  Prioritize what you work on and, as always, build on success. 

BTW I've also used Vermont Naturals for my cat when traveling.  I would prefer you only manage him behaviorally if at all possible, because anything you introduce that affects his behavior will further confuse the issue which already has a lot of moving parts....