This one is hard to explain. The contents of the magazine would be everything that is in the magazine (what it contains.) but if you are talking generally about all of the articles, it would be "content." I don't think I'm explaining this well. In the example you give, I would go with content, singular.

No, it isn't countable, and you can't use the plural form here. In this usage, 'content' means 'material', and it is an uncountable noun.