Hi Jazee,
1) Some cards from Diamond/Pearl were reprints of earlier cards in previous sets before Diamond/Pearl. When the format rotates, the Pokegym should have an updated list of all the cards from previous sets that are still legal to play. On this same idea, some cards still have the chance to be reprinted, thus I usually hold onto my trainers, since they are the most prone to being reprinted. Supposedly, Great Ball, Warp Energy, and Cyclone Energy have been reprinted in the newest Japanese set. So if you toss out your cards (which I would never recommend, at least donate to players that have no cards at all, to learn with), make sure to keep the Great ball, Warp and Cyclone Energy.
1B) They usually don't reprint Pokemon. But in the small chance that a new Pokemon has the same exact Name, HP, Retreat, Weakness, Resistance, same exact wording on Poke-Power/Body, and attacks, then you do not need any outside reference to play it at a premier event. The Card-Dex is a document used when players use foreign-language cards or cards that have a newer version with slightly altered text (Steven's Advice for example).
2) Some cards stay near their value, but many do go down in price. Legendary Pokemon (Legendary Birds/Dogs(Cats?)/Mew/Mewtwo/etc usually keep a little value to them. Same with cards with extreme stats like the 200HP Wailord EX or any Charizard (especially 200 DMG Charizard EX). You can tell from the various people in the Selling area of this forum selling their EXs for $5 a piece, just to get rid of them. I usually sell my non-useful high money cards a few months before the rotation, in hopes of getting a little more value out of them. But I know not everyone can do that.
3) Diamond/Pearl Promos are all playable. Only the World Championship printed cards are not playable. The reason the Lv.X promo cards sell for much less is because they are easier to find. Often time, the non-promo version of Lv.X promos take a hit too because the non-collector players just buy the promo version, reducing the value of the non-promo version. Supply/Demand pretty much.
Hoped this helps, and feel free to ask questions whenever you have them.
