Can Pokemon be Knocked Out partway through completing the Trainer card?
A Pokemon cannot be Knocked Out partway through the Trainer card. You must complete all of the actions of a Trainer card BEFORE you check for game state checks (to see if a Pokemon is Knocked out for example).
Source: WotC Chat (2002-03-06), WotC Chat (2002-03-07)
When you use a Trainer that requires you to shuffle your hand into your deck does that Trainer get shuffled back into the Deck too or does it go to the Discard Pile?
No, the trainer that you play doesn't go back into your deck. {When you played it, you put it down on the table. It is no longer in your hand]
Source: WotC Chat (2001-03-29)
Can a Pokémon Tool be attached to a Pokémon that already has another trainer card (Defender,Plus Power,etc...) attached to it?
Yes you could attach a tool to a Pokémon that had another trainer on it (as long as it isn't a Tool as well).
Source: WotC Chat (2000-12-21)
Last week you said that many cards (Lass, etc.) don't require shuffling their deck if they don't have any cards in their hand (Nov 30, 2000, Q4 & Q15). But, a previous ruling for Gambler said that you DO have to shuffle (May 11, 2000, Q131). Please clarify & confirm. Thanks.
Read every card...on Gambler it specifically states 'shuffle your hand into your deck'. Since your 'hand' can be a 'hand' of zero cards you could shuffle if you had no cards in your hand in this case. However, cards that say trade a card in your hand for one in your deck, and you don't have one to trade; you can't shuffle in that case. Lass says 'shuffle all Trainer cards in your hand back into your deck. If you have no trainers, you can't shuffle your deck.
Source: WotC Chat (2000-12-07)
If I use Pokémon Trader or Energy Search and there aren't any Pokémon or Energy in my deck, do I have to show my deck to my opponent? If there isn't any in my deck, do I get to put the Pokémon Trader or Energy Search back in my hand either?
You don't have to show your deck and you don't put the trainer back in your hand [Ed. note: It goes into the discard pile].
Source: WotC Chat (2000-08-17)
When you use a Trainer card, first you pay the cost (if any), then you resolve the effect of the Trainer card, then you discard that Trainer card. So you could NOT use Item Finder to bring itself back from the discard pile (you would discard 2 other cards from your hand, then pull a Trainer card from your discard pile to your hand, THEN you would discard the Item Finder).
Source: WotC Chat (2000-03-02)