A rifle/load/optics/shooter system that shoots very well at 1000 yards will generally shoot well at 100 yards but one that shoots very well at 100 yards does not necessarily at 1000 yards.

I'm thinking that would be a clear conclusion after reading all that's been shared here.

From the Optics section:
Parallax:
Parallax is essentially an optical illusion. Parallax presents itself as the apparent movement of the reticle, in relation to the target, when your eye moves off center of the sight picture (exit pupil) or in more extreme cases it appears as an out of focus image. It indicates that the scope is either out of focus or more specifically the image of the target is not occurring on the same focal plane as the reticle. Maximum parallax occurs when your eye is at the very edge of the sight picture (exit pupil). Even when parallax is adjusted for a designated distance, there is an inadvertent error at other distances. Most brands of scopes that do not have a parallax adjustment are pre-set at the factory to be parallax free at or around 100 yards; rim fire and shotgun scopes are set at or around 50 yards. Most scopes of 11x or more have a parallax adjustment because parallax worsens at higher magnifications. Generally speaking parallax adjustment is not required for hunting situations and is primarily a feature used and desired by target shooters. A 4x hunting scope focused for 150 yards has a maximum error of only 8/10ths of an inch at 500 yards. At short distances, the parallax effect does not affect accuracy. Using the same 4x scope at 100 yards, the maximum error is less than 2/10ths of an inch. It is also good to remember that, as long you are sighting straight through the middle of the scope, or close to it, parallax will have virtually no effect on accuracy in a hunting situation.
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https://texashuntingforum.com/forum/ubbthreads.php/topics/3371190/1/riflescope-school

Which goes back to load development at distance. Best I've done at 500 yards is 3/4" vertical. Shoot that load at 100 yards, and it stacks then one on top of the other. In the last year, I've load developed for several factory rifles. When they're holding 1" or less at 200 yards, some 1" at 300 yards. At 100 yards, they're making a ragged hole.

That is not to say the ragged hole, 100 yard rifle is going to hold that 1" at 200 yards. That is not an automatic, back to powder charge consistency from case to case. It could look good at 100, but if the load is fighting the barrel harmonics, it is likely to go to poop at farther distances.

Can yes. Not a hard rule. Ammunition manufacturers test loads in various rifles, and find one that seems to do well in most rifles. Even starting from scratch with a given combo for rifle A, B, and C. There is a good chance the sweet spot of powder charge is fairly small. Meaning it doesn't take a wide experiment to find where each rifle shoots.

As an example, an old war horse, the .308. Give me one with a 20" barrel or more, and I'll bet you a dollar it shoots a 178 gr, with H-Varget between 44.0 gr and 45.0 gr. Not a ton of testing within only one grain of powder. I've tested this at 500 yards with mass production rifles and customs alike. USUALLY the sweet spot is somewhere in there.

Where you get into the weeds is something uncommon. Last year a 28 Nosler, 95°F, 195 gr bullet, a new powder with no published data. I had no guideline, so I've got to figure things out on my own. It was a full day project at the range (which is also where I load). I found the sweet spot, but this was combo number one. Will the sweet spot be near the same on the next rifle? Only trigger time and documentation will tell.