Dowels will work for standoffs if they are good hardwood. I use plywood or metal. I have some aluminum plate from my old workplace. Lowe's, etc. sell 1in. wide aluminum strips about 1/8" thick. Also washers can be stacked. I would treat the ends of the dowels with thin CA glue once I found the proper length. (You could even glue on a layer of firberglass.) That will soak the grain and help prevent splitting. Don't know about the elevator flexing other than hinges out of line. They could be installed correctly, but out of line. I had to put braces on mine because the landing gear fell out and hit the tail. Not much wood in the built up tail so I installed braces to help my repairs. Had to glue blocks in the framing for the screws. As far as the CG, I would go wit the calculated CG until I flew it. You can always shorten the blocks to move it back later. It isn't fun to fly a tail heavy airplane on its first flights. Too tail heavy will make a really elevator sensitive plane. I have a 50cc Thunder Tiger Extra that I used the manual CG on. Made it really tail heavy. The first flight was all over the sky until I got it sort of trimmed enough to land. Fortunately, it only cost me a broken prop and slightly bent landing gear to get it on the ground. I have double checked the manual CGs or calculated my own since then.