3) Can you fly between any two aerodromes and what, if any, restrictions are there to this for private pilots?
Basically yes, but there are some points to observe:
* as already stated, some may be prohibitively expensive
* some may be outright forbidden, mainly military
* perhaps stating the obvious but the field needs to suit the plane, i.e. minimum runway requirement
* some may not fit your requirements, perhaps fuel, perhaps food/drink
As a general rule, it is a good idea to phone any field you intend to fly into. Some even require this, the so-called PPR (much hated by some, though I cannot see any good reason to).