Lightweight Blocks

If it's legal, is it responsible to have a clear vision of the two mirrors?

I think the law will change state to state, but generally it is illegal to have anything that obstructs your view in a car. It is almost never applied if, as the type of Jaywalking.

In the United Kingdom this is not a legal requirement to have a mirror, but you must have two mirrors.
Its a similar bill with the passenger mirror.
If you do not have a passenger, you must have a mirror.

It is legal in the United Kingdom as my pick-up has a box at the rear (installed by the dealership) that blocks the rear view mirror so I have to use the reverse wingmirrors

If you have a functioning pair of left and right hand side mirrors, you can paint your windows black back and remove your inner mirror. Look at any commercial vehicle - there is no rearview mirror.

I hope that this scum is attached. There have been cases where something light like a box of kleenex broke someone's neck when he was thrown forward in an accident.

Legally, you could be cited for obstruction. However, this law is never applied.

Many vehicles, including my Truch, has no direct view of the rear
That's fine, as long as I can see what happens via mirror

vans and trucks have rear view window. mirrors so you do not break the law. But Im sure the police stop you to increase their objectives.

Yes, provided that the vehicle has side mirrors - which are all today.

My escort van had no windows to rear door i doubt it is illegal in the United Kingdom at least