Curses, especially, were routinely farmed out. Executing curses oneself was considered dangerous, like practicing law today. Priests were consulted. Like today.
Very, very often -- indeed, absolutely conventionally -- literate slaves were kept as teachers. Any soldier with aspirations to advancement would need education.
You are really stretching to avoid trivially obvious inferences. Read some history? Maybe a literate slave could help you out.
Advancement could be yours!
Take a step back and look at the claim and counter-claim: guy must have been mad to carve that in stone; nu-uh, could have been delegated. I've given many reasons to doubt this is likely. Now you've got "priests" in the mix (this is suddenly a state function, I guess) to hold up this house of cards? The chain of contingencies your "remarkably obvious inference" depends upon is remarkable. And you are inferring all this from a single piece of epigraphic evidence? Good grief.
Ignorance, today, is a choice. You may dispense with it anytime you like. Or not; "nescio id quod sciam," as it were.
Why is it so unlikely that the aggrieved person carved this himself?
Carving it himself -- or herself, we don't have any indication who, or that it was even a soldier who was aggrieved -- is one of many possibilities.
What is ignorant is to insist only the one alternative, among many, is possible, and that slaves were never ordered to do tedious chores, and that a soldier (or whoever) could not have had a slave, and that said slave could not be literate. All officious BS: what we have is a rock. What we know is, exactly, that somebody carved it.
The sort of literate slave you are suggesting, BTW, would be fit for a patrician and command a price beyond what a provincial soldier could afford. Of what use would such a slave be to such a soldier anyway?
You're really stretching it to preserve this fantasy of yours.