Is it possible to go back to Wellspring (and wasteland around) after I traveled to Subway Town? This is important when I want to get all collector cards, jumps and stuff for trophies.

yes

How?

After doing the first mission for Redstone, head back into the resistance base and go to the hot air balloon where you entered the area. A man named Charon will use the balloon to bring you back to Wellspring.

In the garage, right beside the door you exit to get into your car, if you stand near the vehicle lift, the elevator will come up and you can go down into the base.