Walls – How to repair the voids in this stone wall

Ok, those aren't paving stones, those are wall stones (as the Lowes page confirms) - as in retaining wall stones. The lip is designed to hook to the stone underneath and prevent slipping when pressure is applied from the rear.

Which isn't to say you cannot use them, but you'll find removing the lip much easier if you use a good masonry/stone chisel, like such:

Put the cutting edge of the chisel at the base of the lip, holding the chisel near parallel with the brick surface, and strike the other end firmly with the hammer. The chisel will create a very focused point of pressure which will chip the lip away.

Finally, since its a simple fire pit, use stone adhesive to fix the blocks in place. You shouldn't have to worry about the kind of back pressure these bricks are usually designed to withstand.

Honestly, I would imagine your walls will be just fine with a little water running over them. The stone walls all over the Irish countryside have been standing for a year or two and seem to cope with the wet, cold North Sea coastal weather pretty well, and most of those aren't even set in mortar. ;-)

What I definitely would do is consider what kind of footing the wall is built on. If there's a chance of water running down the wall and digging a trough at the base that undermines the wall, you might beef up the footing and/or build in some drainage.