There are a few things you need to know about your index into the array.
First you need to know your normalized position in a ROW:
normpos = (ndx - 6) % 6 // where ndx is current LED index
That gives you 0 through 5 no matter what ROW you are on.
You need to know which ROW you are on:
row = ndx / 6
So you can tell if you are on an even or odd numbered ROW:
rank = row & 1
If you want to go left:
next = rank ? ndx + 1 : ndx - 1
If you want to go right:
next = rank ? ndx - 1 : ndx + 1
If you want to go up:
next = 11 - (normpos * 2) + ndx // EDITED 4/11
If you want to go down:
next = ndx - ((normpos * 2) + 1) // EDITED 4/11
You need to specify your desired actions for moving away from the edges.
TESTED