Can word "sad" be dropped? Would it be correct to say Spiste du da jeg ringede?
I'm not a native speaker, but "sad du og spiste" sounds jarring and odd. Can du be placed after spiste?
I'm a native speaker, and it's safe to say. No, you can deifinitely not put "du" after "spiste" :-)
Would that alter the meaning, or just sound nonsensical?
It's simply just nonsense to say that. It would be like changing your question into something like "That alter would the meaning". That's kinda what it would sound like to a dane.
V2 languages can be jarring sometimes.
What are V2 languages?
But... that's got nothing to do with V2 position, as it's a question, where the verb is in the first position anyway
Can one give an answer like "Det er søndag, jag lå og spiste"?
What's wrong with 'phoned'?