If you 2 3DSes, it is possible to transfer from Gen IV to Gen V.
- Here are the steps in order to transfer from a Gen IV game to a Gen V game:
- Place the desired Pokemon you want to transfer in your Gen IV (HeartGold, SoulSilver, Diamond, Pearl, Platinum) game's PC Box.
- Visit the Poke Transfer Lab on Route 15 in your Gen V game (Black, White, White 2, Black 2.)
- When you're ready, start the Poke Transfer System in your Gen V game.
- Once your Gen V game begins searching for a Gen IV game to transfer Pokemon from, take your other 3DS, insert your Gen IV game into the slot, and select "DS Download Play" on the Home Menu.
- Select the Download Play for DS rather than 3DS once it's loaded.
- Wait until your 2nd DS detects the Gen V game. A Pokemon B/W/W2/B2 icon should pop up. Tap it. From there, you should be able to pick 6 Pokemon from your Gen IV game's PC and transfer those to your Gen V game.
- You will then engage then engage in a minigame in which you pull back on a cross bow with the stylus and release it to send a Pokeball flying to where the crosshairs are aimed on the top screen. You'll have a time limit to catch all 6 of your Pokemon, but don't worry, any uncaptured Pokemon will be put back in your Gen IV game's PC, where you can re-transfer them and try again. There is no limit to how many times you can use Poke transfer; you can keep transferring packs of 6 Pokemon until you have transferred your desired Pokemon from any Gen IV game! You are also free to transfer less than 6 Pokemon in one transfer if you want to.
In no way should you need wifi to transfer from Gen IV to V.
As for your second question, it is impossible to transfer Pokemon from Generations I and II to Generation III games and newer generations, since the stat calculations in Generation III got a complete make over to make Pokemon's stats easier to manipulate. You can transfer Pokemon from generations no older than generation III.
Hope I helped!
Edit: Not only can you use 2 3DSes, but you can use any 2 systems from the DS family.