mipmap blur/flickering at grazing angle, help plz!!

Hi, I have a painful issue about mipmap texture, for Oculus Quest 2 development.
blur/flickering at grazing angle is causing big problem. Would like to seek help from the wisdom of the folks here.

A bit context:

I’m using projected photos for simple background evnironment(in following images). The problem is, mipmap gets too low res on grazing angle grounds. If I force mipmap to increase resolution, then other areas get too much resolution and start to fliker when viewed. especially worse viewed in the headset. 

Trying to understand the mechanism behind it, here are the question:

1 How does UE4 determine mipmap resolution?

A. Is it per triangle based?

B. Does stretching polygon along one axis affect the same way as proportional scaling?

C. If I turn off or use bias to increase resolution of mipmap, some nearby areas get too high texture density and start to flicker. In this case, other than separating geo manually with differently-scaled texture segments, is there a better solution?

D. Is there a way to manipulate the density threshold of the mipmaping?

2 How to visualize the texture density effectively?
I know there is “Required texture resolution” to help visualize the density, but when switch into this mode, even selected the texture , it’s all black. Any one had the same issue?

Many thanks to any light shining into the confused soul!
Best regards