Ignore Steve G's answers.
1. X is for eXchange, it was built for the train station. 15AE was another possibility. (15A, for some reason, wasn't.)
2. The speed limit was 35 for a reason. I'm sure that same reason caused the traffic. If an accident causes congestion, the SL drops in advance of the accident, so that you're going slowly before you hit the rear of the backup. Once the congestion clears, the SL goes back to what it was. Sound traffic engineering policy.
3. I've asked this same question - instead of 3/3/3/3, you could fit 7/7 or even 8/8. The reason is that above at most 4 lanes, traffic flow is no longer nearly as efficient. A single 7 lane road doesn't do much better than a 3/3 divided road. You have to move all the way to the right to get to your exit, and traffic would weave all over the place (drive the Parkway at 6/6 for an idea of what it's like). It also lets cars be separated from trucks if they so choose, which many drivers prefer.
4. Interchange 6-8A widening.