I think that caddies predated etchings on sprinkler heads, vis a vis giving the player a precise knowledge of distance. My understanding is that they had caddies, even in Ross and MacKenzie's day.........
Also, having and actually believing the distance are two very importantly different things.
I've always been really annoyed by all the "traditionalists" who claim that yardage markers on sprinkler heads, strokesaver yardage books, GPS on carts, etc. are somehow bad for architecture because golfers know the correct distance. I've always believed as you do, that caddies provided that information, or at least were capable of doing so -- I'm not sure if any caddies in Ross' day knew you were 158 yards or even close enough to say "about 160" or not. But certainly nothing stopped them from measuring distances from various landmarks quite exactly and writing them down in a book for those golfers who cared. In a windy place with lots of ground game like TOC in the hickory days, maybe knowing 158 yards didn't matter much, but I'm sure it did on some courses, or some more analytical golfers could have insisted on knowing and their favorite caddie would comply even if he laughed at the guy in the pub after the round with the other caddies.
I agree though with your point that having and believing are two different things. And the relative permanence of sprinkler head and published yardage book yardages means any mistakes are long lived. My home course rebuilt part of the 18th fairway five or six years ago, and the first few months after the course opened I hit three shots at a back left pin that flew long, kicked off the slope, and went OB. The next time I played after that experience I was intending to step off the 150 yard marker, but started walking from the 200 yard marker. I counted 57 steps from there to the 150, and 43 from the 150 to the 100. Oops! I've seen errors of a similar magnitude at other local courses, though the ones that cause one to come up short by 7 or 8 yards can got unnoticed for years because there are many reasons why that might happen. I guess going long and bouncing OB several times within a couple months a sort of "in your face" wakeup that makes one want to discover just what the heck is going on! I even found a wrong distance (luckily before I played my shot) at St. Andrews New, I believe to the left of the 10th fairway where one distance in the strokesaver near my ball looked out of kilter from the rest and based on how far it was off I'll bet they accidentally lasered the hill behind the green on that one instead of the laser target that was being held up in the "exact" center of the green (another source for error I'll not get into)
So does this leave an architect room to deceive and confuse the player? Hell yes it does! I won't buy an illusion that makes the green look 50 yards closer, but if its a bit more subtle I might start to doubt the yardage total I'm working from since I know it might be wrong. Even if the yardage is correct it might drive me crazy and make me step it off a few times, and if the terrain doesn't allow that I'll just always be a bit indecisive on that hole. I really don't see how the situation is materially different than it was in the Golden Age. When most players played by eye they might buy a 50 yard deception, but their games were not constructed to require such exact distances as our fly it high and stick it game does today. And when playing the ground game, either out of necessity or strategy, distances still don't mean a whole lot today. I might want to know if I'm 190 if I'm trying to hit some low punch into a gale, but it isn't like I'm trying to look at the terrain and fly it exactly 145 and have it roll 45 yards, I'm pretty much playing it by feel. The distance is just an input into the thought process that controls how hard I hit it, but if my brain was more accustomed to estimating distances, it wouldn't really be necessary for that type of shot since there's plenty of randomness inherent in such a play so having the distance off by +/- 10 yards isn't a big deal.