If you redirect energy to fill in holes in the pattern you must remove some from one part of the radiation pattern to fill in the other. This is a dipole which is half wave length long and makes it about twice as long as quarter wave antennas. No, it must be used as manufactured. If it is cut to fit, it will be destroyed. Yes, but keep it as far from the metal frame as practical.
Any metal alters the characteristics of the tuning and pattern in unpredictable ways.
Each airframe installation is different, so we recommend that you use tape and experiment with locations until you are satisfied with the operation. If you are using two, stagger them on opposite sides and try to have about 24 inches or more in the stagger. This design has been in operation since 1981 but in response to consumer demand they are now being produced for the non-metal airframe market.