I agree with T.G., this is certainly a fight with multipath. If you can not mount any higher, for whatever reason then there is no point researching it... As I looked at it, it appeared that you would get substantially better signal from the stations on Queen Anne, Capitol Hill and West Tiger if you could get above 35' AGL.
I would expect the multipath problems for KCPQ would be less also. That would also make me try a single antenna solution, with KCPQ on the back side of the antenna.
So, if you are limited to the existing mounting AGL, two antennas make sense. You don't need the gain, but the directivity of an
Antennacraft Y10713 makes it my choice for a dedicated CH-13 antenna at your location. (Directivity is a primary antenna quality useful for combating multipath.)
For the Seattle stations, I agree with the Winegard 7496P already suggested.