High tackles have too much potential to cause skull, neck and spinal injuries, you can't tackle without binding (taking full control of body weight). Same reason you can't dive in two footed over the ball in football. Penalty tries are seven points, always have been.
Not quite, you used to have to take the conversion, though it was a gimme in front of the posts. They did away with that a year or so ago in favour of the 7 points. The yellow card was a bit harsh given it only ended up high due to the dive for the try line, but meh, it's a decision and not the reason England lost or anything. A bit unlucky maybe, no more.
Did you see the double red card for the joint spear tackle in the U20 England - France game? Well done referee.
As for the Six Nations, Ireland are worthy winners, and it won't do England any harm to get a boot up the arse at this stage.