Personally, I am not a great fan of individual unions being allowed to set their own laws for U19 games. IMO it causes confusion when we don't need it, because what applies in my neck of the woods could be totally different to another Ref just over the border, and even worse ... when it comes to the internet, there are no borders as this forum proves day in, day out. Eg in this thread alone, we have people from England, Wales, Switzerland, New Zealand and Australia contributing.
The WRU Pathway notes are very good, and I thought I knew them, but although I have a quick glance at them before every U19 game, you can get caught out. The ironic thing when I asked one team to drop a player to equalise numbers, was that the coaches knew an off-duty Ref on the sideline and asked him if I was right. Sadly, he confirmed that I got it right - it turns out we were both wrong. Luckily for me, the side disadvantaged by my mistake were stuffing the opposition, and won comfortably anyway.
Luckily, the WRU Pathway notes can be summarised quite easily - there are only 4 or 5 variations that we need to remember for each game. I assume it's the same in England.