I just got done installing a "short shift" kit in my car, and am familiar with the area you're describing. What I don't understand is what your mechanic means by the part being stripped. Take a look at the 968 parts catalog MG 7 page one, or search the PET for shift lever and you'll see what I mean. There is nothing to "strip" as it's only a c-clip and a groove in the intermediate shift lever.
If you rounded off that groove with one hard shift, you're either one strong dude or your intermediate shift lever already had problems. If it's the lever, try to find a used one. I've never heard of one failing before.
Can you get it in gear while the car is off? If not, I am thinking clutch or shift fork related.
When you replaced the shift lever did you get all of the connections tightened. Sometimes those two bolts holding to the tranny and shift lever are a bear... If that c-clip wasn't fully seated, it would be easy to move it with the mechanical advantage of the front shift lever.