Koln probably does not have much chance of avoiding the relegation this season. And as you said, the Bundesliga 2 is also full of strong teams. So it is going to be hard for whichever team that gets relegated to actually get the promotion again.
Its very hard to get back from the second german league into the first german league , there are so many teams that trying this now for years.
The most problem is that the most or many players will leave the team when you goin down into the 2 league.
Mainz does still have a chance though. The problem is they are going to play against Dortmund and Wolfsburg in the next two matches that are left for them in this season.
I guess even there is a little chance for them that they will be relegated.
Dortmund and Wolfsburg are to big for them at the moment and as i remember they will need 2 wins for the last games.