Thanks for the report. I flew again Monday. There was not as much of a cross wind as before and I experienced a gentle tail wag only once during two flights. I agree that at this point it's not anything to worry about and no need to invest in a gyro. I'm up to 10 flights and have my Wren SS set to 16 lbs. After a couple more flights I'll give it the full 18 lbs. and re-evaluate the tail wag.
I'll have to say that this is one of the easiest planes to land, just like a .40 trainer. With full flaps you can land at about 5 mph. I did catch a gust of wind on one landing that lifted the nose up, then set it down a little hard but I was luckier than you.
The reason for down thrust is to prevent the plane from climbing during acceleration. Also with no down thrust if you trim it for level flight at moderate speed then it will tend to climb when you go faster. The amount of down thrust you want is a function of how big an engine you are going to use. Most guys are using 5 degrees.
Based on discussions in this thread at the time I was building I made my mount for 4 degrees. With my Artes Bee (12#) it was fine. Then I put my Wren SS on it dialed down to 14#. That also seemed to be OK. After bumping the engine up to 16# it had a definite climb at higher speeds. I placed washers under the front to get approximately 5 degrees, which is better. I'm guessing that I'll need about 6 degrees when I give it the full 18#.
I don't see any reason not to put in some downthrust. As you alluded to, the other advantage is to help keep hot exhaust away from the horizontal stab. The other alternative would be to mix a small amount of down elevator with your throttle.