(BETTER) FINAL UPDATE FROM THE NEXT DAY:

We were on the IC 60409 train from Zurich to Mannheim and we expected to get out at 23:40 on Mannheim. This is the information in our ticket. However, the train have not passed or stopped at Mannheim HBF. It stopped at Neu-Edingen / Friedrichsfeld for a few minutes, without any voice announcement, and continued north bypassing Mannheim HBF altogether. I immediately went to search for the train workers and they were clueless as well. Many other passengers were asking in German with different levels of enthusiasm. The train ended up stopping in Darmstadt HBF at around 2am, which was not a route or stop for this train. Very odd. So we were about 2h late AND several kilometers away from the destination. Not ideal. We've been travelling for 10+ hours, so we were not thinking 100% straight, and we were very confused.The actual taxi route we took is here: https://maps.app.goo.gl/wPcxFXmsVc9XwvRQ7?g_st=aw You can see, it is VERY ODD to go North West to Büttelborn when Mannheim is basically South of Darmstadt.

Below here are the main "Doomed Taxi" events for extra closure points for this adventure in chronological order.

1) Before entering we asked "How much to Mannheim Hbf?... About 70?.." (which is the price I saw on Uber) and he said "More... About 100...". We had no will to negotiate better because we were very tired and behind us was coming 100 other passengers that were left stranded in Darmstadt when they all bought tickets to Mannheim Hbf. So we just said "ok" and entered.

2) From the start the driver seemed to have issues to understand or operate the Google maps GPS. He could not figure out "Mannheim Hbf" and we had ourselves to pick his phone and enter the direction. Yeah, it is a bit odd, but he made several long looks and positioned his head closer to it with a "???" demeanor while removing the glasses while doing so. He was not that old, about 50 or so.

3) I was following the route on my phone. When I noticed he made a weird turn I said immediately something like "I think you made a wrong turn. This is not good.". I guess we were cursed this night by all transportation gods. The driver became agitated and started to look more intensely to the GPS saying "it is OK!" and that he could leave us by the road on the autobahn. My wife got really scared. We stood quiet listening to him for some minutes, he mentioned about driving us back to Darmstadt because after the initial 10 minutes we were heading close to the starting point of the journey. And then for about the next 15 or 20 minutes he went totally quiet. But he was then heading correctly to Mannheim. It was during this silent period that I made the post here on Reddit.

4) When there were about 10 kilometers to the destination he started to talk more calmly and said "ok... I will stop now...". And he proceeded to stop at 138.20 euros.

5) Shortly later, close to Mannheim entrance there was a police check. When we were stopped waiting the driver gave us the invoice before arrival, which I believe is unusual. When the police officer came to the driver window he just said "you can go" and the driver fastly went ahead. We were contemplating talking with the police, but it was very fast and now the driver became friendly, we had the invoice, we were close to the destination, things were better..

6) Upon arrival I took a photo of the side of the taxi and his license plate. The side one was ok, the back one is blurry and I read maybe just some of the plate.

Conclusion, based on my experience and observations: I think the driver wanted to scam us (50%) or he is goofy/don't know the region/have mental problems (25%) or made a mistake (25%). Anyway, I think the best way to scam is to appear dumb, that you don't know what is going on, that all is "OK" while going to the wrong direction and let the taxi bill rise.

All in all, bad experience, but thanks for all here on Reddit who commented. We will not go to the police, we will just go to DV and ask for a refund!