A1. The drivers are signed AFAIK but as I explained the enforcement features of 8-10 are much higher than previous versions.
A2. Fact that they did not install at the time of the IDE install is often subject to settings with the windows install itself. I cannot see your system so from here its almost impossible to tell why.
A3. In the grand scheme of many many thousands of users there is a minority who don't seem able to install drivers and the reasons I gave prior are just a few of those.
A4. You are not running "device manager as an admin but installing drivers. I think you may be a little confused there. Google has lots of ways to turn off driver enforcement from temporary to permanent.
If you computer has a policy in place that you cannot change (educational or business computers etc.) then you should seek out your IT departments help with that. A policy is often put in place to protect a system from abuse in one way or another.
A5. There is no definitive fix as the reasons as stated can be multiple. Each fix is a case by case.
A6. It is not limited to Arduino as you can find large amounts of problems for multiple types of hardware from Mice to almost anything else you can attach to a computer. Continually getting drivers signed by MS is not a cheap process which is why so many bits of hardware don't or only go a partial route.
Arduinos like any similar board does expect a small amount of basic knowledge of both computers and electronics.
If you are looking for a reason to return the kit I am afraid I cannot give you one.
Also take note that we are almost exclusively users like your self in these forums giving freely of time, effort, and experience to help others. It can be a thankless thing at times but we keep coming back.
Bob.