Only way to avoid grout filling deep crevices on the textures is to tape over them for grouting. It was my experience, using scrabble tiles, that the paint came off and i had to re-paint the letters, which was not a big problem. I have two concerns about your project. (1) Weldbond is a water-based adhesive, and will rehydrate when wet. I know there are a few on the forum in the past who have claimed they used Weldbond for outside projects, but I wouldn't take a chance. Since you're using Hardibacker - a cement board, why not use thinset - a cement-based adhesive? (2) Your panels w/be very heavy, so my concern is that the screws might pull through the board. If you use enough screws, be sure and use the hardward - like washers - to attach.

Will this be inside or outside? Welbond would be okay inside. If you break up tiles, just remember that there will be sharp edges, especially any porciline tiles, so you may want to tumble them, we use rock tumblers, but if it's a big project, a cement mixer has been used to tumble stuff!

I was thinking thinset as well-and its probly pretty good for kids to work with.
For your letters-If you took regular glazed tile and used a dremel or other similar tool to engrave the letters into it, the grout would then fill it and show up the letters! an advantage to grout filling the space. I have seen others do this with different shapes like spirals to make an accent on a tile.
Maybe a french cleat could be considered for hanging-its one of the best ways to hang heavy objects and its very easy to remove and then replace the panel. you can google it. Good Luck!