IANAL, but it seems to me like the US could totally pass laws regulating Germans on German soil. It just couldn't enforce them in US courts (short of kidnapping Germans from German soil, or luring them to the US somehow). So it would be mostly pointless but not impossible.

Also, multiple countries have laws claiming universal jurisdiction. As I understand it, French laws against genocide denial claim to apply universally.

From your own discussion, it seems like "subject to the jurisdiction" should be understood as "can be judged in court". Diplomat is immune = can't be judged = not subject to jurisdiction. Immunity lifted = can be judged = subject to jurisdiction.

And also, depending on the crime (and acknowledging it is easier in some cases than others), countries have often agreed to prosecute the diplomat in their home country.

There's also the fact that Diplomatic Immunity is also a lot narrower than people think. Consular Officers are generally not covered by it, but then you get in to the de facto. Okay, NYPD sees consular plates and doesn't ticket a vehicle. Or tickets it but there's no enforcement, because "maybe it's the Ambassador's vehicle". And then a city, state or country is generally not going to make a scene that affects ambassadorial relations over a few hundred, or thousand, dollars of parking tickets.