"...Interestingly, you don't normally say 'a tea' for a cup of tea in the same way. I am a British English ( New Zealand) speaker..." Believe it or not, people do say 'a tea'. That's how you'd order a cup of tea at Starbucks. 'I'll have a green tea'. Outside Starbucks, I can't imagine saying that.

Most of the time these are interchangeable in common usage. "Would you like some coffee?" (Referring to the substance, uncountable). "Would you like a coffee?" Is really short for "would you like a cup of coffee" and refers to the cup, (countable). I think this is a fairly modern usage. If you are referring to coffee before it is made up (beans or ground coffee), you would always say 'some coffee'. "When I go to the shops I'll buy some coffee'. Of course, here it is uncountable (or in the case of beans, practically uncountable!) I think that's the key - "a coffee" means a cup of coffee, so you can use it whenever you might say 'a cup of coffee' . I can't think if an occasion when some coffee would be wrong. You might say "i tried a nice coffee the other day" - here this is short for 'a nice type of coffee'. Even here "some" would sound fairly natural. Interestingly, you don't normally say 'a tea' for a cup of tea in the same way. I am a British English ( New Zealand) speaker.

It depends on the sentence. If you say; Coffee is bad for you. Coffee comes from Brazil. Can I have some coffee? the word 'coffee' is uncountable. It is a singular noun with no article. However, if you say: 'There are many different coffees for sale in that store', you mean different types of coffee, so in this case it's a plural noun, and therefore countable. and if you say: 'How many coffees did you order?' you mean 'How many cups of coffee?', so here again you are using the word as a plural noun, and it's countable. So you can say either: 'Can I have a coffee?' (countable, meaning a cup of coffee) or 'Can I have some coffee?' (uncountable, meaning an unspecified amount of a liquid) So, as you can see, either is possible, so long as the internal grammar of the sentence is consistent.