I actually think things are much safer nowadays with cell phones and satellite communicators, and the wealth of information on the internet about what parts of certain countries are safe. You can even message random people who actually live there on social media about the situation on the ground. Many are happy to reply.
Up until the Russian invasion of Ukraine it was fairly straightforward to do a train journey from London to Singapore. Other than Russia and Belarus the entire rest of the route (London-Paris-Frankfurt-Warsaw and Ulaanbaatar-Beijing-Nanning-Hanoi-HCMC-Siem Reap-Bangkok-Penang-Kuala Lumpur-Singapore) is extremely safe in terms of violent crime.
Warsaw-Moscow-Ulaanbaatar was also safe for tourists prior to the Russian invasion.
(Nitpick: The -Siem Reap- segment would have to be a bus due to the lack of functional rail in Cambodia. However, China is building rail across Laos to connect China and Thailand by rail)
Getting from London to India over land is a little more involved. The European rail network will get you to Turkey comfortably and safely with very little effort (-Frankfurt-Munich-Budapest-Bucharest-Istanbul all have regular trains), and Istanbul-Tehran(Iran) rail service also exists, but heading further east will send you into some unsafe areas very quickly. In the absence of the Russian situation you could do -Warsaw-Moscow-Astana(Kazakhstan)-Almaty-Wulumuqi(China)-Kashi and then as long as it's summer/fall you can take a bus from Kashi to Gilgit(Pakistan), then another bus to Islamabad, and then you can take trains from Islamabad-Lahore-Delhi(India), which travel through some sketchy areas but also isn't a war zone and you'll probably be just fine on the train. Once you're in India you once again have all the rail you want, you can continue to the far south of the Indian subcontinent by train.
I will agree that most of that is acceptable if you stay on the train. Lots of spots I wouldn't want to get off the train, though!
And what good will your satellite communicator be? They're a very good safety precaution when you're heading away from civilization, but this isn't an issue of danger due to remoteness, but danger from the people and politics along the road. Not to mention that your inReach is illegal in China.
Different issues here. It might not help you against politics, but it may help you in "lawless" type situations where cell networks get taken down by gorillas (e.g. if you were on the ground in Ukraine during the invasion)
As for China, it's probably technically illegal, but in China intention matters a lot in law enforcement. If you're doing a cross-continental trip and just passing through China and have it off during that time they are unlikely to actually care. The 4G/5G coverage in China is excellent anyway and violent crime is rare.
Yeah, as a tourist I wouldn't expect too much of an issue. I've seen it multiple times that I get something of a pass on security measures. I've been routed around metal detectors more than once, almost never asked to take a swig from my water bottle etc. So long as I wasn't in a sensitive area I wouldn't worry much about it. (Although the part of China I would be most interested into going into the sort of location where I would be carrying my inReach is Tibet.)
China concerns me for political reasons, not for personal safety ones.
Up until the Russian invasion of Ukraine it was fairly straightforward to do a train journey from London to Singapore. Other than Russia and Belarus the entire rest of the route (London-Paris-Frankfurt-Warsaw and Ulaanbaatar-Beijing-Nanning-Hanoi-HCMC-Siem Reap-Bangkok-Penang-Kuala Lumpur-Singapore) is extremely safe in terms of violent crime.
Warsaw-Moscow-Ulaanbaatar was also safe for tourists prior to the Russian invasion.
(Nitpick: The -Siem Reap- segment would have to be a bus due to the lack of functional rail in Cambodia. However, China is building rail across Laos to connect China and Thailand by rail)
Getting from London to India over land is a little more involved. The European rail network will get you to Turkey comfortably and safely with very little effort (-Frankfurt-Munich-Budapest-Bucharest-Istanbul all have regular trains), and Istanbul-Tehran(Iran) rail service also exists, but heading further east will send you into some unsafe areas very quickly. In the absence of the Russian situation you could do -Warsaw-Moscow-Astana(Kazakhstan)-Almaty-Wulumuqi(China)-Kashi and then as long as it's summer/fall you can take a bus from Kashi to Gilgit(Pakistan), then another bus to Islamabad, and then you can take trains from Islamabad-Lahore-Delhi(India), which travel through some sketchy areas but also isn't a war zone and you'll probably be just fine on the train. Once you're in India you once again have all the rail you want, you can continue to the far south of the Indian subcontinent by train.