Hi Kelvin,
I'm working on my Impact and going through all the same problems. I was missing the same tubes and I'm waiting for a new package. Hopefully it will be complete.
I don't think you really need a datum line and it really doesn't matter how high or how low it is. The main thing is that it is parallel to the stab. My plan is to follow their instructions and level up the stab and put the wing at plus .25 degrees. From what I've heard the down thrust is proper if you line up the spinner back plate with the front of the fuselage.
I'm a little concerned about using the top of the vertical stab to align the wings. It sure looks like the top of the vertical is warped a little. I think I will try to measure to the bottom of the vertical.
I've got the added bulkhead tacked in the tail and am trying to figure out how to get glue on the front side of it. A long tube I guess. I'm also planning on a crutch from that bulkhead forward to the trailing edge of the wing.
Keep up the good work. If I had read your thread earlier I would have saved some time putting in the landing gear support. I bet I had it in and out 25 times before I got it to fit.
Regards, Jim O