What kind of time delay switch to choose to reverse motor for just a couple of seconds
The simplest way is to use the drive mechanism itself as the timer. Have a DPDT switch which reverses the motor when it hits an obstacle, and arrange the coupling so that the switch doesn't turn off again until the mechanism has backed off a bit. A micro switch with a long lever arm may do it.     

If that doesn't provide enough delay or clearance then you will have to use a timer. This would be switched on by the obstacle detect switch, then operate a DPDT reversing relay for 2~3 seconds. Here's a suitable timer:- 

Kemo 113A Time switch

Depending on how your existing relay is wired, you may be able to use it for both jobs. The timer outputs would be wired together so that the relay turns on (or off?) to reverse the motor if the obstacle timer is running or if the down timer is finished.