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If these are new plantings and they are showing stress along with yellowing leaves; it is likely a watering issue.
A consistent watering regimen is needed immediately following planting in order to ensure that your new tree remains healthy and adapts to it's new location.
These articles will help you.

https://www.gardeningknowhow.com/ornamental/trees/tgen/watering-newly-planted-tree.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/what-is-well-drained-garden-soil.htm
https://www.gardeningknowhow.com/ornamental/trees/oak/how-to-grow-an-oak-tree.htm

Most likely the tree is declining because of root disturbance. There shouldn't be anything under a tree but mulch out to the dripline. In one photo, there are a number of shrubs under the tree that someone must have removed to make way for the rocks, causing even more site disturbance. If you purchase the home, I would suggest moving the rocks away from the trunk. Vegetation, even grass, under a tree takes nutrients away from the tree. One of the worst things a person can do is suffocate the tree roots with raised beds under a tree, which is a popular design. Hopefully with correct watering and fertilization, the tree will recover.

This is a sensitive question. An arborist will probably tell you that it is not safe to remove superficial roots. Not only are they necessary to feed trees but they anchor the tree so it doesn't blow down. Oaks have a particular problem, oak wilt disease. No pruning or cutting of roots should take place when leaves are on the tree. This is because the disease that causes oak wilt, a fatal disease, is active in warmer months. Bark is like skin: it protects the tree from infection (and pest infestation). The Univ of Florida has good information that I hope you find useful.
http://hort.ufl.edu/woody/root-prune-guidelines.shtml

You are right on time. It takes about 20 years for oak trees to start producing acorns. Peak production doesn't occur till between 50 and 80 years. A single tree can produce 10,000 acorns in one year. However, bumper crops don't usually happen every year, more like every three to five years.

Here is more:
https://extension.tennessee.edu/publications/Documents/W126.pdf

Uh oh. This is a massive problem. You will want to have tree professionals remove that tree immediately before it affects others in the area.

Sudden oak death is not able to be treated and spreads to other trees very rapidly.

This article will offer more information: https://www.gardeningknowhow.com/ornamental/trees/oak/sudden-oak-tree-death.htm

If you are only doing some light shaping without removing a great deal of foliage, then it can be done any time. But for more extreme reduction of top and sides, I would wait until spring when new growth can cover the visual effects of radical shaping. But it's primarily an aesthetic issue. This tree is cold tolerant so I don't think the exposure of stems will risk frost damage, which is why with some plants it's best to leave the leaf cover to protect the woody stems over the winter.

https://landscapeplants.oregonstate.edu/plants/quercus-ilex