1. You can set the rumble how you want it. You select the DS game you want to use rumble with (the game has to support the rumble, you can't make any game you want use rumble.
This is a full list of all the games that use rumble), press Y, then Y again to go into that games settings, then set "Rumble strength" to off, low, middle, or high.
2. I've had mine for about 5 months and it's still working perfectly. Some people claim that theirs came with a dead battery, but mine was fine.
3. You can have as many games as you can fit on your MicroSD card.
But the EZ 3in1 has three memory slots:
One is for your save file for whatever game you last played (that relies on the battery so that's why I set it to backup my save as soon as I turn my DS on).
The other is the quick, temporary memory which it loads a game to when you want to play it, but as soon as you turn off your DS, it's gone again (not your save, just the rom, but that's saved on your MicroSD anyway). That memory can fit most normal GBA roms in, but a few games are too big, so they have to go to the third memory.
The third part of the memory is bigger than the other, and it's permanent, so the rom will stay there even when you switch the DS off. But it's really slow to copy a game to, and you don't want to keep changing the game on it all the time becaue it does slowly wear it out.
So basically, for nearly all the games, you just press A an play. But for a few games that are bigger than normal, you still just press A, but it'll be really slow at putting the game onto the 3in1 becaue it has to use the bigger memory. But next time you play that game, as long as you haven't put a different big game in that memory, it'll load instantly.
Sorry for the long post, I hope it's not too confusing.