Short answers, pulled from the story.
Constitutionality is the quality of being acceptable under a supreme law. A law, procedure, or act is constitutional by default unless a legitimate authority declares it unconstitutional through a legitimate mechanism.
The United Kingdom and New Zealand have no codified constitution against which courts can test legislation. The Netherlands and Switzerland have codified constitutions but do not grant courts the authority to strike down laws on constitutional grounds.
Swiss voters can petition for a plebiscite, which forces a direct public vote on whether a piece of federal legislation should stand or be repealed.
Two categories apply: a politician acting outside the powers of their constitutionally-established office, and any government action that prevents an individual from exercising a constitutionally-protected right.
Yes. Statutes enacted by subordinate-level legislatures, such as those of a state or province, can be declared unconstitutional just as national legislation can.
In many jurisdictions, a supreme court or constitutional court serves as the final legal arbiter. Its rulings on whether a law or official action is constitutional cannot be appealed further.