You need downward extension on your ECB to trigger landing while going upwards, that's why AI is only possible on frames 3-4 of uair. For reference, this vid by @
schmooblidon describes it nicely at 4:40 (For some reason I couldn't get the url with correct time)
Also yes, you can hold shield all the time provided that you press down at the correct timing. But I think it's still better to time the shield because you then get the drop even if you misstime the down input. The correct timing for the down input also varies depending on which frame you land on. On the other hand you don't lose anything if you time the shield when you time down well, so it's just better for consistency. If you try to just time the down input, you have to hit down on exactly frame 4 (Uair 1 being frame 1) to cover landing on both frames 3 and 4 of uair. With my suggested method, you can input down frame 4 or later, though even frame 5 means 1 wasted frame if you happened to land on frame 3 of uair.
If you are consistent with just timing the down input, then that's good, and no reason to change. I'm just giving my opinion of which method I think is the easiest to learn.
For whatever reason, the notation <4 for autocancel means that the move will still autocancel on frame 4. It makes no sense to me.