I'm not sure where you got your information from, Oko. I only buy one box per set. In HP, I only received 1 EX and that was Mew EX. However, since these cards are originall printed on sheets and spliced up by machine, I would think it would be odd that Nintend would just make a sheet with lots of Mightyenas and Crawdaunts, and only one or two Mews. The resale value is higher for Mew due to it being a demaded Pokemon and that few people really want to resell Mew cards due to it being a demanded character.
Rarest trainer card - In my box, it was the Holon Explorer(?). Incidentally, I received the reverse holo of that card. Rare Candies are a demanded trainer card, but due to the several releases and the new variation of the card, there are more copies out there than some of the one-time release trainer cards.
However, Shiny(star) Pokemon are only released 1 per 3 boxes. It's been like this since the intial Shiny release in the EX sets(Team Rocket Returns). However, I've been fortunate shiny wise and in the last two boxes I purchased(Legend Maker & Holon Phantoms), I've not only received both Mew EXs, but a Shiny Pokemon in each box.