No, you need the pronoun then: ?מה הם אומרים
There's no pronoun here because you're not talking about anyone in particular, just in general.
It's what you say to kids when they're supposed to be polite and say please and thank you.
No. And also the translation is right but very misleading. This is a sentence without a subject. It's equivalent to a passive sentence. maybe a better translation would be "what should have been said?". If I try to find a context for this sentence, I imagine a mother that tries to educate her child, so she gives him an ice-cream and asks him "מה אומרים?" and expects the child to answer "תודה" ("thank you").
It is a present progressive tense and is used when you say what is doing just now. If this type of a sentence is about a general statement, you have to use a present simple tense.