you can't catch qwilfish in emerald. the only gba Pokemon game that I know has qwilfish is fire red.
However to the answer stated above you can catch Qwilfish in Pokemon Colloseum, purify it, and then trade it to Emerald via. a GNC/GBA connector cable.
Yes, You can catch Groudon in Pokemon Emerald.
You can't catch one in Pokemon Emerald.
No you cannot catch Entei in Pokemon Emerald.
You can't catch a Bellsprot in Pokemon Emerald. You trade it from Pokemon Leafgreen!
You can find it by using a Super Rod on Iron Island.