These are great questions! Thank you!
I have generally run combat on a grid, but not always. I use words like adjacent, near, far, and very far when I’m not on a grid. I will be sure to include that in the Arbiters principles that we’re working on (I added it to my notes). For speed what I do if, say, someone is slightly slower, I’ll say “you don’t quite reach your destination because your speed is lower”.
All this being said, I feel like the combat can be improvised on the grid without any issues. There’s no need to worry too much about a perfectly balanced encounter.
Hope this helps!