The Danes wouldn't need to sew shrouds for the Geats if Grendel triumphs because it would mean that all the Geats had been killed by Grendel, and there would be no bodies left to bury and therefore no need for shrouds.
The narrator describes Grendel's hatred for the Danes as "pain as raw as knuckles."
Geat; Danes
Grendel terrorized the Danes for twelve years before being defeated by Beowulf.
Beowulf killed Grendel and he was a Dane, so yes
12 years
Beowulf is a legendary hero who travels from Geatland to help the Danes by fighting the monster Grendel.
Geat; Danes
The Danes.
Good vs. Evil
Grendel is not a monster , it is a "tribe" living in pre danish "Denmark" defending their land against danes, invading from the east "sweden", it might have been rather difficult, and the danes was helped by their allied "geates"
Grendel haunts Heorot for twelve years before Beowulf arrives to help the Danes.
The battle between Grendel and Beowulf takes place in Heorot, the hall of the Danes.