What you're asking for is fairly unrealistic. The only way this could work sustainably would be for something to exist where you host your own tile server and routing service and patch that into OSM. Otherwise, even if the app itself is open source, the backend will cost money to run and will be proprietary.
The reason that OSM is able to be fully open source is because you host the tiles on your phone and do the routing calculations locally.
Not under 50MB but there is: https://organicmaps.app/
And
https://osmand.net/
I find Organic Maps best for driving and OSMAnd+ best for walks.
No
The current FOSS offerings do the calculations on your device, so you'd need the maps downloaded locally. The small apps that stream their tiles from OSM/Jawg/ESRI/Mapbox etc. don't support navigation because of this
Not FOSS but the closest thing you'll get to this is GMaps WV on F-Droid, made by the DivestOS team. Even that does not support navigation though, it only provides directions (usable for me, your mileage may vary...)
I like organic maps cause it does shit locally, offline. It has to download the maps though which is certain to be larger than 50mb. What you are looking for is a cloud based maps solution like google maps and you aren't going to find that in foss space
