I can provide some insight on the equipment side for the USA XM21s and USMC M40s...
I've talked to several Vietnam (Army) snipers when researching my project rifles. Though they didn't have the gadgets, they knew their dopes. Also, the Army guys had a decent selection/training regime in-country. The platforms/ammo allowed good probability for a first-round hit within the normal engagement range (usually dictated by terrain and visibility) that was usually well within 600Y.
The 308/30-06 rounds were more than sufficient for engagement to 800Y, and the rifles (when they were in good condition) were as accurate and fast to use as some of today's systems.
The challenge was in the optics and their bases. I've shot the AR-TEL, M84, and Redfield greenie scopes. The auto-ranging function on the AR-TEL and the ranging line on the Redfield greenie made engagement relatively easy to 600Y. On the M84, the fat post makes shooting past 500Y really, really hard (its easier to shoot iron sights past that range than with the scope). Adjusting windage on any of these scopes isn't very easy, and I'd suspect many just held for wind.
The critical flaw on the XM21 was the scope mount. The XM21's ART-TEL mount was made out of anodized aluminum and had one screw and an slot on the side mount to mate with the receiver. The slot on the side mount was usually a little too big for the receiver's slot. Contact area was lower than it could have been, so most of the pressure went into the single screw. The mount had a nasty tendency of loosening under fire and would eventually become so loose that it had to be discarded or welded to the receiver. (I was so disgusted with mine that I asked Mike Sadlak to modify it to make it fit better.) Most of the other metal fasteners/screws in the ART-TEL base are low grade steel. Also, the AR-TEL scope sits really high on the rifle relative the comb of the stock. None of the Army snipers I talked to remember building up the stock with a pad--they said they learned to shoot with their chin high up on the stock.
The M84/M14 mount was more durable because it was a lot simpler (based on the M1C mount). It was fielded in limited quantities and with little support. The M84/M14 combination wasn't successful because the scope offered a lot more complexity with few advantages compared to the M14's iron sights.
The critical flaw of the M40 was its stock. Unlike the resin impregnated wooden stock of the XM21, the untreated wood stock of the M40 tended to warp under the high-humidity and heat in Vietnam. The guts of the greenie scope were also susceptible to shocks and heat. The rifle just wasn't that durable--as to be expected since it was an improvisation. Senich's book states that Marines would paint their M40s in order to seal them a little.
Skill and experience with the platforms could extend the engagement range well beyond 600Y if other situational factors permit. With my M40 replica (real M40 barrel, real M40 mount, real greenie scope), I can't elevate past 850 and have to hold the rest. (Nothing radically different from a 1990s-2000s era M3-Ultra Lupy equipped M24.) On my XM21 replica (SAK NM barrel, ART-TEL scope/mount), I can use the bottom of the stadia box to hold to 1K, but the thickness of the lines makes precise shot placement (F-Class 10 ring) hard. On my M40/XM21 replicas, I'm shooting M118LR, so I even have a slightly hotter round than the Vietnam shooters.
This doesn't answer the skill side of the equation, but the challenges our Vietnam snipers overcame makes their efforts even more impressive.
I have a question then on your AR-TEL. Mine is two screws that attach to the receiver and they are both anodized(?)/parkerized(?) steel. Definitely not aluminum. As is the crossbars and base-plate. The weakness in mine, as I see and as I was told, is a little bit of dust getting between the cam and baseplate. That will throw off the simultaneous elevation/power increase as you range.
I was told when I bought this it was an 'original Leatherwood'. So far as the way I see it made, it's pretty good quality.
As you noted though, unless someone got custom knobs made there is no windage adjustment.