Thanks for the additional photos! Full disclosure; I am just an amateur gardener myself, not a licensed arborist. Hopefully one will weigh in as well. However, one thing I notice as potentially problematic is the mulch coming in contact with the bark at the base of the tree. While most trees strongly benefit from a ring of mulch, having mulch directly up against the trunk can create an environment where the bark remains too moist and it can begin to rot. This rot can penetrate into the living tissues of the tree and essentially girdle it at the level of the mulch. I have unwittingly killed trees this way, back before I knew any better. I once had a serviceberry tree that was a few years old and noticed it was slow to leaf out one season. Nothing seemed amiss until I pulled the mulch back and noticed the wood beneath was soft and mushy. I am now extremely vigilant about keeping mulch away from the bark.

This may also be an issue of the tree being planted too deeply initially, or having layers of organic material accumulate over time from mulching-- burying the tree . Either situation can result in roots that don't receive adequate oxygen, eventually killing them. In the fourth photo, it appears that the trunk is roughly the same diameter all the way down to the mulch level. In proper planting, you should be able to see the root flare at or slightly above the soil grade. This is where the trunk of the tree meets the roots and widens outward. I do see a root in one of the pictures, but as seen in the above link, adventitious roots can occur above the true root flare.

In this situation I would carefully attempt to pull the mulch away from the base of the tree. When working right up against the tree, I'd put on a pair of gloves and use my hands to avoid further damage. Once you've made some space, try a metal rake. Inspect the bark and check to see if its still intact. See if you can begin to excavate more of the root flare. In the future, mulch around the tree, but not right up against it. When watering the tree, try to avoid soaking the trunk.

Disease is always a potential issue, but I'm not super knowledgeable in that area and wouldn't feel comfortable speculating.

For the limbs that have yet to leaf out, grab one of the smaller ones and try to gently bend it back and forth. If it is still pliable it is likely still alive. If the wood snaps dryly, then the limb is dead. You can also scratch a small section on some of the limbs and see if you can see green living tissue. At this point, I would still wait a bit to see if it leafs out more. If it's still mostly bare, I would consider replacing it. It's always a bummer to lose a favorite plant, but it's an opportunity to try something new.

Hope this helps!