How can I extend the rough-in drain away from the wall?
Yes, the solvent welded elbow above the trap does makes things challenging. How excited are you to completely remove the old trap and elbow?

It's likely the connection can be made using the existing trap. Swing it a little to either side, cut the old riser short, and use a pair of 45° elbows to make a jog in the riser to align with the new sink tail piece. Or, use a flexible section rather than fixed elbows.

If completely removing the old trap and elbow is your preference it can be done mechanically or with heat.

The mechanical method is to cut off the elbow at the base of the socket. Next, grind or sand away what's left of the elbow on the pipe. Take care not to cut too deeply. A rotary tool such as a Dremel works well.

The heat method is to warm the fitting with a heat gun. At some point the PVC makes a transition from solid to rubbery. When it reaches that point it can be massaged and tugged with a wrench or maybe a gloved hand. This breaks the solvent-weld bond between the fitting and the pipe inside. The pipe temperature will not lag far behind the elbow temperature, though, so it takes some care to work the elbow off without deforming the pipe too.