I have been installing windows since 1998 and the last 10 years in leadership. I have lead crews and worked on crews installing Marvin Infinity as well as many other similar products. I can tell you that install instruction have always been available, for Marvin Infinity, since they came out with the brand, twenty years ago. Definitely more acceptable in the late 2000's. Although, even if you had found them back then, no manufacturer tells you exactly all the methods to use and not use when installing windows. You can certainly install frames without sashes and no instructions will tell you either way is best. There is definitely more than one way to install a window. Not ever as black and white as the wrong way or the right way. There is no true one way to install windows/doors.

Can one man install large windows on his own, yes, as you witnessed. I fear that instead of having admiration and respect you got worried and turned critical. Finding help to show up every day is a very hard thing, especially when people do not want to pay well for installs (more the dealers then home owner). Many dealers will charge a big amount for the install and then pocket 50% or more of that figure. They end up paying the installers sometimes under $100 for a small to medium size window and under $150 for larger windows. The only way to make money is to "slap" them in and never spend time solving little issues. The ones that care too much to do it perfect actually lose their shirts and move on to other trades or careers.

A window can also be a but out of plumb. true or level and still be a good install with a good operating window that will have no effect on warranty. For many years I worked as a Service Manager for one of the largest window companies. I tried getting an exact "out of tolerance" spec from higher ups. They never wanted to publish anything concrete, but I was so surprised what they allowed. Their main concern, as far as warranty, was that if the window contacted all seals, operated correctly and locked correctly, it was covered under warranty. I would never install windows/doors that far out of whack but there is room for give there.

I have seen many windows manufactured out of specs and only a handful too bad to not install. Then there is the opening you are installing into. Openings and walls out of plumb or not true are another challenge. The bigger the opening the worse. The OP didn't specify how he measured or calculated "many" out of true point. It's a very broad complaint that needs specifics. Go to measure a hundred mansions, and all their windows, and I bet none will be perfect in all level and true points you can measure. Pay twice as much for an install and an installer can spend hours on a window to make it perfect in every way but no one wants to spend the money and what most get now is fine.

Here are the main things to look for as you set aside your level:

Double or Single hungs (DH or SH)- Does the sash travel up and down with no major resistance (twisting in jams pinching sash) and no major side to side play (bowed out jams). The mid point is the easiest place to warp the frame as opposed closer to top and bottom. There should be relatively even spacing on either side of the sash when operating up and down. The sash may move from side to side when operating so keep it even while raising and watch both side. Keep in mind that an out of square sash or frame or frame or both can cause the same end results so don't be quick to blame install.

A racked frame that leans to left or right (or manufactured with a shorter jam than the other side), will show in the sash when fully closed at the check rail where the locks sits. Look at the reveal from one side to the far other end and see if the bottom sash won't sit even with the top. A little off is only aesthetics the no one else may ever notice. Too much and you may start having a harder time turning the lock in and out of lock position. Way too much and it won't lock at all. At the same time, I have seen the locks to keeper in the rear, slightly off and this is the main culprit (even though the sashes are not perfectly aligned).

Sliders/Gliders - They are a lot like SH windows but on their side so most of the same things can be looked for here, but Sliders are much more forgiving when out of tolerance. Typically, when you fully open a slider sash, you can push the sash up into the header and then slip the bottom out towards you to remove sash. There are anti-lift block in the header when near the fully closed position, to keep the sash from being removed unless you fully open it. If the top or bottom (or both) of the frame are too tight the sash may never be able to be removed (especially one you lock it in more with trim and caulking).

Casements/Awnings/hoppers - close and open just like doors but into a pocket in the frame that has tight tolerances. When fully closed and locked, casements seal the best out of all window configurations but also the least forgiving. The sash on a casement will also sag down with time. Some way worse than others, especially when taped glazed. If the install is causing the sash to lean, towards the lock side (away from hinge) the bottom of the sash corner may drag on the frame, causing resistance when closing or making it impossible to fully close and lock. Once again, this can be to a frame or sash or both that are made out of square. it can happen the other way, where the sash leans towards the hinged side. The sash end at the locking side will rise up and drag the frame at the top.

When a casement sash closes fully but you look down or up, in the two corners at the locking side jamb, and see a gap from seal to frame, then the frame may not be true. If one or more sides or corners of the frame are in or out too much, away or towards you, the sash can never seal tight against the frame. This is just as true in doors. Close any door in your home and you will probably see the top locking jamb corner touch before the bottom (or vice vera). Note that the multi point lock system, that clamps down on the sash keepers to lock the sash, can be out of adjustment and may keep the sash from fully closing as well.

Picture windows (PW) - you would think a solid glass glazed into a frame would have no sealing issue and less install issues but the same issues can put stress on the glazing and deglaze some glass. These stresses can also crack glass, especially when the temps change quickly at night or in the morning.

Some issue are not even stemming from out of manufacturing specs or install issues. Windows get bigger and bigger with huge glass in the same frames that were never meant to sustain such loads. Especially since the large you go with glass the more additional support you need to add to the frame and no one does this. This is why vinyl windows have gotten such a bad reputation. Manufacturer specifies to add additional support to the frame if over a certain size at certain points under the frame. This is actually on the exterior and like I said, no one does this. Even the inside frame should always have shims under the frame in multiple spots where the weight bears down but so many don't shim here (or drywallers take out those shims). the frame will warp along the sill and make it seem like it was a bad install. I've seen frames bowing with the weight of the huge glass, right from the factory.

Buildings settling is a thing but pretty rare to the point it makes a window inoperable. Although, casements and doors are more susceptible to this. There are other factors out there that can affect windows and doors, even changes humidity (hi or low) as well as the same for temps. I have seen windows/doors having slight issues. You adjust best you can and solve them, only to have a related issue occur during opposite seasons and temps/humidity.

So, the moral of this story is don't kill your self being overly worried about windows and doors being slightly out of plumb, level or true. A manufacturer will never go out of their way to nitpick an install and void your warranty. It really must be pretty bad for visiting warranty tech to report back to the company that a window/door is so poorly installed that it won’t be covered under warranty. As far as operation, as long as it operates smoothly and locks correctly, and the aesthetics is unnoticeable to a fresh eye looking at it, then just enjoy your new windows and doors.