You can’t travel to Wengen by car. They are not allowed. You would need to park the car in Lauterbrunnen and then take the train anyway.
The trains are very easy to navigate. Especially at Lauterbrunnen, which is the end of the line. You get off the train, walk across the platform 10 feet, and get on the awaiting train.
If you use the SBB app, it will tell you which platform you will arrive at, and which one the next train leaves from. If you miss your train, just get on the next one. The concerns about luggage are valid. Just be ready to get off the train (standing by the door with your luggage) a few minutes before it arrives at your station.
Driving in Switzerland is easy. The problem is that you’ll probably end up getting a train pass anyway, since all the best views require you to go up into the mountains, either by train or cable car.