1) the income from pleasure domes is n*population, where population is colony pop plus 5 from a scientific lv 5 HQ, 10 from an expansive lv 5 HQ and 15 from a scav lv 5 HQ. There is an additional 1 phantom population (I suspect it is an extraterrestrial that visits the pleasure dome some times.) n is the number of pleasure domes + 3 divided by the number of pleasure domes. So at 1 dome, income is 4*pop, at 2 domes it is 2.5*pop and the lower limit is 1*pop.
This means they are justifiably not-bad for you in scav vs scav 1v1s (when you have some source of abundant power) and also in FFAs with a lot of expansives/scavs.
3) The colony is fairly insignificant in MP. In the campaign it grows a little slower if life support is expensive, it is not directly influenced by anything players do but only by the price of construction materials (alu, glass & steel) and life support (water, food & oxygen).