Colloquial Standard English
1.
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?"
-or-
"Then, you were still sittting there?"
-or-
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?"