MKT,
case 1
Impossible in alcatel systems, the TRX band gives the list of frequencies you can choose, during frequency allocation.
case 2
same thing
case 3
there will be radiations, but weaker. The pattern might eventually be modified as well.
The dipoles of your antenna are the size of the wavelength / 2. So putting 900 into 1800 dipoles is possible : the dipoles are wavelength / 4 -> still proportional to the frequency.
But the pattern will be modified accordingly, and I suspect the gain will be lessen (probably divided by 2, as far as i recall).
And it is not dangerous. It is never dangerous 🙂 electomagnetic waves will always find a way to the air (“freedom !” i heard them say, sometimes), and they’re never going to blow up anything.