When I move to the stormcloak winterhold camp to begin the 'liberate winterhold' quest, Galmar Stone-Fist attacks me on sight. He wont let me yield and I can't engage in dialogue with him, and thus can't advance the stormcloak questline.

I think it may be linked to me murdering a hunter in the field beforehand, but it told me all witnesses had been killed so my bounty was off. When I went into windhelm, all the guards began to attack me, but I was able to yield to them. They didn't attempt to arrest me afterward, curiously. I went up to Galmar again, but he continues to attack me whilst the rest of the stormcloaks stand by and watch.

How do I make him unhostile?

Either give yourself a Calm spell, or select him with the console up and type in "disable" then "enable".

Disable/enable didn't work and I don't have acces to any calm spells strong enough for him.

Is there a console command to set his disposition or calm him?

I had a similar problem with the owner of the Drunken Huntsman in Whiterun. He'd attack me on sight, if I stepped out of his shop, he'd chase me outside and the guards would kill him. Found out by opening the Crimes info window, that I had a 5 septim bounty in Windhelm (don't really know how). Went to Windhelm, none of the guards seemed to care. Had to run up to one and initiate conversation with him before they said anything about a bounty. Had all the normal options for bounties except paying the fine wasn't listed, and had 2 options to bribe. One bribe was for 637 septims (for 5 septim bounty, I think not!) and another bribe for 2 septims. After I paid the bounty, the owner at the Drunken Huntsman didn't give me any more trouble; though now he calls me nasty things when I first start talking to him, but then thanks me for my business, lol.
I tried all manners of console commands to remove his hatred of me before this, but none of them seemed to work. Only once I paid the bounty did he finally began acting mostly normal.

The guards have no problems with me. I can't even talk to them properly when I interact, they just spout lines about the mage's college and whatever, like they usually do

Try this -

Open Console and target him then type

resetAI

That should force him to become calm again, and shouldn't attack you unless you have done something after to cause him to attack.

I went to try the above suggestion but, for no discernable reason, Mr. Stone-Fist was cool with me again without me having to do anything. I suppose he needed a little time to himself. Thanks!