the problem is - you want 3 things in a 2 dimensional world - as I understood - and thats tricky without getting confused.
Okay thinking the musicians way - Up to 10 Post IDs (notes) get numbers (1-10) that correspond to positions ( 1|1 etc.) on the map (keyboard). Too complicate for my easy piano player brain. To sightread and play music fast from a sheet you simply need one note and the position on the note grid - thats basically it.
So give people a prefixed grid or map with two-digit-numbers f. ex. for the positions: 01, 02, 03, 04, - 09. This is the upper row from left to right. Next row down starts with 11, 12, 13, - 19. This is for a 100 possible positions on a map. Of course you can cut that down to less positions, maybe a 5x5 matrix which gives 25 positions (11, 12, 13, 14, 15 first row - 51, 52, 53, 54, 55 last row. I think that would be my favorite.
The good thing is: for positioning you only have two things to deal with: Post ID + Position - like: 345-13 or … that will reflect Post number 345 on position 13 on the grid.
Its like driving: you know your car, look at the map and drive to location. Maybe you like my suggestions.
And in the WP-Backend show the map-layout and explain the grid first - and then let people enter their numbers for Post IDs and position.
Aloha from Germany