1. The field pattern will not change .Will remain the same(HPBW for the H and V planes, also the null points and all other parameters) as long as you will not change the environment( moving the antenna from that place to a better one). Remeber that all the diagrams from the datasheets are related to ideal propagation environment(no obstructions, continuous propagation,free Fresnel zone, no reflections and so on…). In the real field there are a lot of buildings,obstacles that will alter the ideal diagram: probably you already saw such a real diagram in any planning tool maps.
2. Of course the coverage will increase but will not double it. Not even 50% of improvment
3.I think that there are no RBS(BTS) that could damage an antenna due to high power. We are talking about GSM Environment and not radar where the average radiated power is like 300W or even more. 20dbm means 0.1W that is something regular even for a mobile. Usually BTS acts with 40-50Watts.