The following are my answers to your questions based on my experience:

1. OS .91FX fits entirely inside the cowl except for the stock muffler (see photo).
2. The landing gear seems to be strong enough for grass runways unless you land off the runway in tall tough weeds. However, it is springy and may occasionally need to bent back into shape.
3. No control surfaces are sealed and there is no flutter. The UCD 60 appears to be less flutter prone than the other UCD's. The early UCD 46 was known to have flutter issues with it's stabilizer and the Giant UCD's wings can explode. My friend can testify to that because he lost his $499.99 Saito 2.20 in a pond when his Giant UCD's wings failed.
3. Grass fields can be hard on wheel pants but I always put them on. Letting the axles stick out through the wheel pants will only make the hole big and ugly due to vibration unless you reinforce the hole with a piece of hard wood or plywood on the inner side of the pant.
4. For the canopy I used RC56 canopy glue and back it up with screws because the glue doesn't adhere very well to that canopy material.
5. I used a 2.25" dia plastic spinner with a metal back plate on an APC 15 x 6 (not wide) prop for the OS .91FX and it works very well.
6. The stock tail wheel was replaced with a Sullivan tail wheel bracket for 10 to 22 lb models and is a perfect fit in the space provided on the fuselage.