How to verify the claim of entropy available made by Linux 4.5?
You can't, and you're not the only consumer. The operating system rips you off. It uses true randomness to prevent attacks by shoving code into random places. And randomly sequencing it's network traffic. But here's the thing...

“We have to remember that what we observe is not nature in itself but nature exposed to our method of questioning.”

-Werner Heisenberg
and that's compounded with the concept of the observer effect.
Essentially what the kernel does is quantize various external events via a made up second order differential algorithm. There's a write up in Documentation and Analysis
of the Linux Random Number
Generator but essentially it comes down to temporal quantization of hardware events as :-

And so,

It seems therefore that Linux 4.5 underestimates its amount of entropy

Yes.  That's the conclusion of all of the academic works I've seen on /dev/random. It's essentially the least signification digit of system time when something happens. But, the problem is that it shouldn't take 160 PDF pages to convince us of randomness. My concept of Open Entropy dictates that it should be screaming right up in your face without funky science.
With delicacy, what's your real question?


Nothing in this post relates to Micro$oft operating systems.

If this is important to you, build your own entropy source. It's not difficult considering the 2nd law of thermodynamics. Get a diode.

There's a dangerous and hugely misunderstood  meme out there that suggests /dev/random = /dev/urandom without appreciating the role of time. $(\epsilon, \tau)$ and the left over hash lemma  is everything.