Can I add an elbow to my kitchen gas line to improve clearance with the range?

Yes, you can do what you suggested, but it would be easier to do the below:

You already have an elbow aligning the pipe with the wall. Just flip the elbow at the wall to the right. Replace the existing cut-off valve with an in-line valve. Clean all the threads before re-assembling. Use pipe thread compound or gas pipe tape on all the threads. After you tighten all fitting. check for leaks.

If the height of the pipe is high enough, you could just give the elbow valve a 1/4 turn down. The flex pipe would attach from the bottom and the flex pipe would be parallel to the wall.

Yes, you can add fittings like a 90° elbow, with these caveats:

• Be sure it's legal for homeowners to do gas work in your jurisdiction.
• Be sure the part is rated for gas.
• Be sure you know how to properly seal threaded gas connections with thread compound or (gas rated) thread tape and good technique.
• Be sure you test with soapy water when you're done.