2. 2 possibilities: there is no _clip there or the tail amor value is really high. I can move up the _clip of the MiG-19 a little bit though. But you'll see the tail sunk into the ground.
3. Currently not possible, I haven't seen any aircraft with more than 3 gears.
2. Place the _clip hooks in tail, not in tail fin.
3. Check Do 335, there are support wheels modeled in tail to protect the prop, and you can use them in take-off to certain limit. You can also use something similar for the tail fin.
The explosion when tail hits the runway is a bit odd since usually the plane just slides on the tail (at low speeds, MiG-19 is obviously too fast for this) or the tail breaks off (when the hit is too hard). Maybe the speed is just high enough to cause explosion, or then there is some bug causing CF damage (explosion) instead of tail damage (tail breaks off).