You can say "en öl" when you would say "one beer" in English. You're talking about one unit (a glass) of beer.
However, when it is treated as an indefinite quantity, it becomes neuter.
In this case, we're talking about beer as something we can't count, not as one (or several) glasses of beer.
I hope this helped.
You've got it. It's the same difference with är and finns. Remember that while "there" can refer to a location in English, it can also serve as in impersonal pronoun, like "it". "There" is especially used in existential sentences, like this one, in the same contexts where Swedish uses finns/fanns.
It was not beer: det var inte öl. There was no beer: det fanns inte öl. It is not beer: det är inte öl. There is no beer: det finns inte öl.