@tim-0 I would use M106 and G4 "Wait" commands in the slicer end gcode.
M106 P2 S255 ; set bed fans to 100% to cool bed G4 S180 ; wait 3 minutes M106 P2 S0 ; turn off bed fans
G4 S180 will wait for 180 seconds before the next command.
As for your 3rd question. I'm not sure how to have an "if" command for temperature, hopefully someone else can help with that.
Out of curiousity, when would your print bed be under 30°C at the end of a print?
Another possibility if you have fans under the bed and a sealed chamber, you can use the fans to move heat around the chamber and increase chamber temperature! Importantly If you do this, run another PID tune for the bed before heating it!