When testing loads at long range can it be assumed that left and right stringing will be wind and up/down is actually group size. It is more convenient for me to test at 300 or 400 yards than it is at 100 yards.
No it is not. If you are testing at 3-400 yds you need to do that on days when the wind is a non factor. either get up early or go just before dusk. Testing for me at 100yds is a waste of time. All my guns shoot in 1 ragged hole there so I don't learn anything. Ask any long range shooter and they will tell ya that 90% of all match shooters at 1K fight vertical every day. The others shoot vertical real bad. No, dont assume anything about verticle being your group. Yes, it will determine the group, no, it's not indicative of what your GUN is doing.
When it gets right down to it, if your gun shot 5 inches high and 3 inches wide at 400 yds, then, your gun shoots 5x3. Go fix that. don't be afraid to tell people what the gun really does in real world conditions. You'd be surprised how many people who shoot 2Inch rings at 400yds who can't shoot as good as You at your house on any given day.
Wind is "relatively" consistant. At the 1000yd range last year I shot in a ton of wind all the time and I shot 14" wide when someone else shot 7 in the same relay. Perhaps there was only wind at my bench and target but if wind is the whole picture, then there is only 7-10 inches of wind variation over 10 shots that take me aprox 1 minute to shoot. That's a good rule of thumb. Unless you happen to be shooting in a hurricane, There's not that much condition to fight. There's just people who are better at fighting it than I am. Your gun at 3-400 yds shouldn't vary more than the above mentioned 10" and if it's so windy that you get more than that, shoot another day. If that's every day conditions, get better in it.