The "left" filter for time left. If I set it to "2h" does it take all less into consideration like "30m" left?

No, only what the blizzard API considers as 2h (which I guess effectively may be anything between 2h and 30m, 30m excluded). If you want to include 30m you have to explicitly add both: or2/left/2h/left/30m

Ok perhaps I do not understand the notation. Is this correct:

bid-percent/20/or2/left/2h/left/30m

If I want to search for bids below 20% market with 2h left or less?

Yes, correct.