Adding some more points in the same format.

Hybrid navigation
Isn't that just "rotate map according to heading" ?
I can image this to have two configuration options:
- active: always/in navigation mode only/never
- heading source: magnetometer/direction of travel (IMU, some custom hardware, etc.)

For the record I plan to support this in modRana on the MapBox GL native widget (once usable)

Navigation in tunnels
What you are describing is basically dead reckoning or inertial navigation you take an initial known position and then try to compute position fixes later on via data you have available (absolute speed, compass, acceleration, etc.).

Ideally this would be something generic that works with the smartphone sensors (accelerometer, magnetometer, gyroscope, barometer) or any other available hardware (data from car sensors). This could be part of the location API, or something sitting on top of it, like the map matching API recently added to OSM Scout Server. That way all location using application could use this, possibly without even being aware the position fix came from inertial navigation and not actual GPS data.

Yes, in some cases having data about an upcoming tunnel & it's properties could help with the estimation, but then the logic would have to be much more wired to the app & it is still questionable if it would really help.

On a related note, do some of the commercial navigation applications, especially those integrated inside cars (so supposedly having all the car sensors at their disposals) do something like that (eq. GPS less position estimation) ? That could help answer the question how hard or even doable this is.

Multiple routes
I think the idea is you get multiple routes during route search, possibly even from multiple offline/online sources. Then you select one, forget about the others and follow it, including rerouting. (Yet another thing on the modRana TODO Matterhorn. ).