I think I figured it out (still please correct me if I'm wrong). I was almost right with my original approach. My mistake was very basic and (dare I say it) stupid.
I have been trying to squeeze 600 lines in one inch: 300 blue lines, 300 red lines.
What I should have done is halve the amount of lines per colour: 150 blue lines, 150 red lines to give me 300 ...
You can try to use mock location indeed after you enabled them, have a lock here: https://www.xda-developers.com/fake-android-location/
Also won't be possible to download the map so that can be consulted offline?