Another Brit. It's all in the context. "Quite" can easily be a strong superlative but it relies on shared understanding between the parties so I wouldn't depend on that interpretation.

It's fine in a spoken context as tone can add sufficient extra meaning. If you can guarantee the listener is expecting an informal register then it probably also comes across as a superlative. "I met him and he's quite unpleasant" would usually imply he was a total c*nt.

Not really, but it depends on context. The nuance is often difficult to detect when written. When spoken, a phrase like "quite extraordinary" or "quite brilliant" would usually be a superlative, as you suggest, but it would usually be indicated by emphasis on the "quite". And the adjective itself already has to be quite strong, so "quite good" will never really mean "great".

This is classic British understatement. It doesn't literally mean 'great', but depending on how it's said it can be interpreted as such.