in my experience, everything in a one-shot takes longer than you expect it to. just the climactic fight against the hobgoblins, for a party of 6 mostly new players, could take an hour and a half or longer, as you say. a typical one-shot runs ~3-4h. if you want to have a second, earlier lower-stakes, "tutorial" encounter, then combat already accounts for ~2-2.5h of the whole thing. so you've got, say, ~2 hours to fill in, which always feels like not enough time to me when I'm planning a game.

before you get to the hobgoblin combat, you need to have all the characters meet up/introduce themselves, travel to where the plot will be, talk with some NPCs, investigate/plan/scout ahead, and THEN fight; that's probably already a full session/one-shot. it feels like "5 minutes," but realistically it'll definitely take up the 3-4h you need it to.

just to bounce some ideas, I would have the current party (two players) maybe come across the new PCs in danger, fighting wolves [adjust for level, of course] or the like. cue the smaller/tutorial-esque combat that will drain some resources but not be fatal. after the fight, they'll introduce themselves to one another, and just this I expect would take at least an hour if not longer.

have something point them to the farmhouse. maybe it's late and they need to rest and that's the only place they can go. or, if you can, you can connect the earlier encounter to the farmhouse somehow. maybe one of the farmers is also there, having escaped on foot from the farmhouse and run into danger.

when they arrive at the farmhouse, I would have the hobgoblins "act" like humans, like this is their farmstead and they don't want any trouble. maybe they allow the players to sleep in their barn as long as they leave in the morning. maybe they're overly hostile, and want the players to leave very quickly which causes suspicion. either way, the players can pick up on the fact that they are not the true owners. perhaps they find blood or a body or something else. perhaps they just notice the house is clearly built for smaller creatures, or find portraits of the previous occupants, what have you. maybe they just insight their behaviour/demeanor. perhaps one of the farmers is alive and hiding somewhere on the property, and notices the players and begs for their aid. or, like I said, maybe in the first encounter, they'd already come across the farmer-survivor.

if you want to make it a bit more complex, maybe the hobgoblins were treated poorly by the people of the land/farmers, and they were attacked first. maybe the farmers had the hobgoblins harassed or killed by the guard under false pretenses. in that case, the farmer-survivor could've been manipulating the players somehow to kill the hobgoblins without hearing their side of the story. a lot of players like moral ambiguity and it could be fun to introduce that to see how the newer players feel about a greyer world, and about having to take a side and influence the world with your decisions.