Time Travel in Shadow is quite straightforward - since time flow varies between Shadows, to go back in time all you need to is go to a Shadow with a slightly slower time rate, while to go forward you just need a Shadow with a slightly faster time rate. You are unlikely to notice te difference, as even if you go back to the Age of the Dinosaurs the difference is a tiny fraction of the history of the universe.
states that As the amount of time-traveling you do increases, the probability of Hitler winning World War II approaches one.
This is absurd, as the Germans were so badly outmatched. The British Empire was strong enough to withstand the Nazis without allies; when France fell, one comic ran with the text "So we're alone then? Yes, all 500 million of us."
The only comparable powers on the planet - were the USA and USSR, whom Hitler also declared war on. No minor time travelling meddling could prevent the Nazis being crushed.
But...Amberites don't have to make minor changes. Maybe the default is that all Earth-like Shadows are controlled by the local Hitler (-99 pts, 1pt owns Shadow), but ours has had major Amberite modifications from the norm.
Suppose that when Flora decided that Earth was an interesting potential home, she decided that uniforms by Hugo Boss weren't enough to justify the Nazis and decided to get rid of them! She's not a warrior, so rather than mucking around with military matters she would be better off simply increasing the resources available to his enemies.
Start with the British Empire: in the 19thC the Empire was referred to as "an oak tree in a plant pot" (by Gibbon?) - a reference to how bizarre it was that to vast an empire had come from such a tiny island. Flora could have increased the resources of the Empire - people, raw materials, factories - by a factor of 5 or 10. A side effect of this (probably deliberate, given she lives there) would be the expansion of the American colonies, allowing the creation of the USA: which has two important effects. Firstly, the massive wave of colonization which provides the US with a massive population - a population that would otherwise have been under Hitler's control. Secondly, the USA acquires Alaska from Russia, reducing Russia's overextension and debt: strengthening the 3rd of Hitler's enemies.
Alaska is important for other reasons - for one a land border between the USSR and the British Empire would increase the likelihood of a war between the two: just as Stalin helped Hitler carve up Poland, he might help carve up the Empire - before becoming the main course himself (just as Poland had helped Hitler conquer Czechoslovakia before being invaded themselves).
Also a Nazi invasion of the USA is no longer absurd if Alaska can be used as jumping off point. It's worth considering that the Bering Straight might not exist without Flora's intervention, allowing a rail line into America, protected from the extreme cold by mountains to the north.
Another major change she may have made is to technology. The classics of Nazi superscience - robots, sound cannons, intercontinental bombers https://en.wikipedia.org/wiki/Wunderwaffe
could face constant problems are the laws of nature are changed against them, while Allied technology - from radar to nukes - is benefited (probably as much to ensure that Flora has TV and movies as for the military advantages).
Finally, the war saw a massive increase in women in the work force and triggered the social changes of the '60s: again, likely deliberate on Flora's part to make Earth more to her liking.
would Flora go to all this trouble? My thought is that she likes the defensive advantages. Her Shadow Earth is surrounded on all sides by Nazi police states; anyone, particularly any male Amberite, it going to be hassled by SS units determined to stop them - a layered defence that requires fighting your way through wave after wave of machine gun wielding monsters.
Meanwhile though, anyone in
Earth using Shadowshifting is in for a nasty shock!