1. According to this old post I dug up:
The formulas used are shown on the Environment tab of the Economics window
Surface Temperature in Kelvin = Base Temperature in Kelvin x Greenhouse Factor x Albedo
Greenhouse Factor = 1 + (Atmospheric Pressure /10) + Greenhouse Pressure (Maximum = 3.0)
So every gas adds a little to the greenhouse factor but greenhouse gases add 10x as much.
my takeaway is that any (nontoxic) greenhouse gas is interchangable with any other, and likewise for any (nontoxic) non-greenhouse gas.
2. Terraforming an atmosphere can melt surface ice, which will cause a sudden discontinuity in your surface temperature as the planetary albedo changes. I think it's reversible if you decide to cool off the planet again for some reason.
3. In general, not all planets and moons can be terraformed into perfect habitability for a given species. For standard humans in Sol, you can't terraform Saturn's moon Titan to colony cost zero. That's where genetic modification comes in handy.
For the planet you posted, it looks like you will hit the maximum greenhouse factor (3.0) before you get it warm enough for baseline humans. By my estimation that planet would need a greenhouse factor of 15 or so.