And some Adventure Time talk of the past few eps...
Finn losing his arm has been foreshadowed for literally years. Since everytime you see alternate versions of Finn in the future or a different universe he nearly always had his right arm covered or it was robotic or whatever. The grass sword basically sealed the deal last season that it was coming. Plus it's definitely not the last we'll see of Finn and the grass sword being that he was able to do things not seen before in when he was trying to stop his father from leaving and the flower over his stump does give hints of that.
As for Finn's father? Pretty sure that is him considering it goes with the theme of the show of deadbeat dad's who aren't ready to be father's (Marceline, Flame Princess). Plus he was jailed for a reason and Finn already has an adventurer, rightous father; his adopted father.
For all the talk of Adventure Time being a stoner cartoon or whatever, the writers do seem to know what they're doing and I wouldn't be surprised if this turns out to be the last season since they keep saying they eventually want to end it.