Colloquial Standard English
Is this colloquial English? Is it also Standard English?
"You were still sitting there then?"
2. In other words, does the term "Standard English" include colloquial forms?
I don't know. Standard English would prefer the inversion and without the filler then, perhaps with so: SO, were you still sitting there?
I don't think inversion is mandatory (especially in US spoken English, where the tone would clearly indicate the question):
"You were still sitting there?"
"Then, you were still sittting there?"
You were still sitting there, then?"
<Standard English would prefer the inversion and without the filler then,>
So it's not Standard then?
I think it is proper and formal to say it this way. The "then" is replacing the meaning of "at that moment" or "afterwards" but referring to a past moment.
Think of the sentence in present form; say talking to someone on the phone and you couldn't believe that she's still waiting for the table:
"You are still waiting there now?"
That looked like perfectly standard English to me. Nothing colloquial about it.
Sure that's fine to say. But I'd probably say: "At that time (or moment) you were still sitting there?"