Do you say 'facial tissues', or Kleenex?

Just "tissues".

Too many syllables, and not nearly as catchy.

Sure, DVR is an annoying TLA. My parents refer to their DVR as "the box". (The verb is then "record it [on the box]".)

If you don't count supercomputers as being mainframes, they're still good for that too.

The litre is just another word for cubic decimetre; since the metre is defined in terms of the speed of light and the second (which in turn is defined by atomic properties), and not in terms of the kilogram, the litre will not change volume. Similarly the fluid ounce is defined as a fraction of a gallon; the gallon is defined in terms of cubic inches; the inch is now defined as 25.4mm: again the metre is the only dependence. So again, even though fluid ounce sounds like it should depend on mass, in fact it is a unit of volume and so behaves like one.

Yes we do. We just don't have a constitutional document that you can point at and say "That's the constitution" like the US does.

The Government, or to give it its current full title, Her Majesty's Government of the United Kingdom of Great Britain and Northern Ireland (there's a clue there—the Government belongs to Her Majesty, so she is not a part of it), consists of the ministers, not the monarch, who acts on the advice of the ministers/government.. The word government is also sometimes used casually to refer to all of Parliament, which again sits under the monarch.

I imagine this is historically related to the emergence of Parliament and the Government from the gradual weaning of power away from the monarch, rather than the from-scratch construction of a system of government. If it makes you feel better, think of government in this context as a homonym (or a polyseme if you prefer) with the word that is used elsewhere.