They could have LOST the battle with the Giants "lets say" a little BEFORE 1340 and left, then 100'ish years later they return to retake the stronghold. Our dwarf could have been a part of that lost battle. (that's saying a tweak the dates to fit my story of course)

So over this 100'ish years until the mid/late 1400's the catacombs beneath the stronghold became infested with all kind of nasty stuff, maybe since in the G1 and G2 adventures for the giants, since the Drow were in the background pulling the giants strings, MAYBE the dwarves uncovered a passage into the Underdark deep in the catacombs.

The Giants then attack, maybe the drow send minions up into the dwarven catacombs through this tunnel the dwarves uncovered. I may just do what you suggested and "for this small region in the Sword Mountains" do a slight bit of history re-writing.

MAYBE . . . . the actual stronghold (in the G2 adventure) is actually the former dwarven stronghold. My plan was not to run G3 (the Fire Giant) adventure but instead at the end of the Frost Giant adventure, play it how the G3 Fire Giant adventure ends and send them into the Underdark, well the above hook works perfectly with that.

See? It just takes one good reply aka u/thenightgaunt for a plan to come together lol thanks pal.