After reading all the above replies I realised that may be the inheritance problem can be split in two aspects: heir's legal right for bitcoins and physical ability to get it.

The legal part depends on jurisdiction and is hard to predict, implement for all possible cases and keep up to date. The physical part is purely on protocol layer and universal around the globe.

The system can be modular and consist of two layers accordingly. The physical layer should be implemented as a set of applications or services.
The legal part support can be in a form of some kind of plugin or may be not present. So many jurisdictions can be covered gradually step-by-step or implemented by third-party as if plugins in modern browsers.

It is up to heir if to take funds before legal approval and have troubles with the consequences or wait for permission. Still physically trustless but legally optionaly dependent and location specific.

It looks like in that way many of the questions above can be solved.