Porii to answer your question about players deck knowledge, while you're looking at the cards, you're correct that's your own knowledge, since it states to look at, not reveal. However because the card says that the rest get discarded, at that point the cards go to the discard pile, and the discard pile is knowledge allowed by BOTH players.
Now, as for an actual ruling, I would say that because it's Lugia Legend's power allowing it, just because your opponent may not know it, Lugia Legend knows that it's there, (kind of like the Yanmega question from the begining of the post, Yanmega takes into account everything BEFORE the Giratina discards it, it's knowledge the pokemon has.) And so, because Ocean Grow says "Once during your turn, when you put Lugia LEGEND into play, you may look at the top 5 cards of your deck and attach ALL Energy cards you find there to Lugia LEGEND. Discard the other cards. I see two things here, one, it says ALL Energy cards, and since Lugia LEGEND knows they're there, they have to go on. Second, there is a period seperating "All energy...Lugia LEGEND." and "Discard the other cards." Meaning you HAVE to attach ALL the energy cards there first. Once you've done that, THEN you discard the other cards. If I was juding at a tournament, this is how I personally would rule, because of the two reasonings given, still though, you should ask the HJ or have your friend ask before hand.
Pokepop, Using the same ruling as before I would rule this way in regards to your question. The cost to activate Junk Arm is to discard two cards. The exact text says "Discard 2 cards from your hand. Search your discard pile for a Trainer Card, show it to your opponent and put it into your hand. You can't choose Junk Arm with the effect of this card." Note the period. First you discard the cards. Then, (and the one thing that drives me crazy about this game after having played Yugioh for so long) is that you do not need to be able to fulfil the requirements of the card in order to use it. If you discover no trainer cards in your discard pile, Junk Arm fizzles and goes to the discard pile. However, because the cost to activate Junk Arm happens first, if you discard a trainer I would say that since now the resolution is happening, you can take back the other trainer that isn't Junk Arm that you used to activate it. Again, ask a Head Judge at a tournament before doing so, but that's how I would rule.
Good luck all!