(for pure speculation and reflection)
Your 2nd example is such impressive poetry, with the strong parallelism between the two lines. In the Dag yig gsar bsgrigs, it makes a distinction in meaning whether the tshad is used after a verb in present form or one in the past. I think it can work in both cases if you just translate "However much [there is or was]." And sometimes "No matter how much [there is or was]" which has a different ring to it.
Dear D, I would also think that in theory, it would work with all three tenses. With imperative, it would not. “Whatever I did/do/will do” .... D.