Hi,
1) Do you mean population inversion? This just basically means the Er ions (or any type of atoms/energy state) are largely in the excited state, so that stimulated emission/spontaneous emission are favored/possible.
2) The gain will noticeably drop since less photons will be absorbed (EDFA’s absorb in the 980 nm range). Try it yourself with OptiSystem and using a sweep!
3) If you want to change the band drastically you might need to use a different material or a Raman type fiber. Although there are some effects like temperature which can broaden the spectrum.