The short answer, VC, is that they don't
necessarily become countable-- it is often the speaker's option/conception that makes the choice.
1. (you have no '2')
| In the countable case, can "an education" be any education such as primary school, middle school, or University? |
|
-- not likely; more likely it is e.g. 'an adequate education'.
| How big attendence did the meeting have? |
|
-- the question would also take the article: 'how big
an attendance...?' And yes, if they have an indefinite article, they are singular countable nouns.
And in your a) and b), as I said above, in many cases the speaker has the options:
high attendance or a high attendance
poor attendance or a poor attendance
great support or a great support
Some are better than others (in my estimation) but all are possible here.