Why didn't Albert's dad just tell Captain Loki why he needed 40 guineas in exchange for Joey? ("I can't save my home with only 20 guineas.") The dad should've considered that someone might take pity on him and his family, and thus give him the money. It would've been less difficult (and more honorable) than arguing for a price and making up all those compliments that he doesn't even know himself if they're true.
Because this would feel like begging, it would hurt his pride to beg and get some pity and because dealing out the price for the horse has a more "honest" feel to it - money you earned in one way or another instead of it being a gift of pity. (Believe it or not, pride is a big motivator in people's doing.). Also, consider that Joey was sold to the Army, not to Nicholls himself. Nicholls could not simply go and spend Army money for charity. And since 20 guineas are no cheap sum - and 40 even less - I doubt he had this sum as a private person.
It's implied that Nicholls was aware of why Ted was selling Joey. When Nicholls rejects Albert's request to join the army, he says this line to Albert: "Your father's done what he had to do, you know that". Whether Ted explained it to Nicholls before they discussed a price or after they came to an agreement is anyone's guess, but I do agree with the above response about Joey being bought with army money rather than out of Nicholl's own pocket. I would also like to add: it should be obvious that Joey was not the only thing Ted was selling to save the farm. It just wouldn't make any sense. Ted bought Joey with 30 guineas in the beginning of the movie, and he sold him to Nicholls for 30 guineas to save the farm. If Ted only needed 30 guineas to save the farm, they never would've needed a horse in the first place. He could've just saved that money to pay off Lyons later. Therefore, it's safe to assume Ted needed more money than 30-40 guineas and was selling multiple things to save the farm. Nicholls likely understood Ted's desperate situation, but also understood that Ted could sell more things than just Joey, so he thought that Ted should still convince him that Joey was worth they money he was asking for.
Why did Albert's friend just stand there and let the gas kill him? Last time I checked, you're supposed to be intelligent in order to make it into the Army. Why didn't he hold his breath and close his eyes, and then rush forward until he found the junction that Albie was going into?
It was World War One, intelligence testing for soldiers was barely coming into being, and that was only in the US Army. Elsewhere they were taking pretty much everyone they could get who was able-bodied and wasn't actually known to be mentally handicapped. (In-universe, they took Albert too, who in the play is illiterate and in the film, though literate, seems a bit simple.) Inability to act decisively in a trench filled with mustard gas isn't a matter of IQ, in any case, I figured he was pretty stunned and IIRC we never do see what he did after the gas came up. Also, while not breathing in mustard gas would help it burn less of your mucous membranes, and the severe blistering burns mustard gas causes are not immediate, by the time he was aware of the gas, taking a deep breath to hold his breath would still mean breathing it in and choking on it anyway. Even if he didn't inhale, mustard gas needn't be inhaled to take its effect on the membranes of the eyes and nose. (It also takes more awareness than you'd think, when frightened and disoriented, to remember to hold your breath in the split-second before the air is too full of gas or smoke to breathe. Speaking from experience with a house fire.) I can't remember if we see his friend immediately fall or if it's implied he dies later, but he's hardly an idiot for dying in a war with no gas mask in an unfamiliar part of the trenches. He just had really bad luck.
Also, World War I is the first time mustard gas was produced on a mass scale and employed in warfare like that. You can't expect people to have intuitive understanding of how to deal with a gas attack in a time when the concept of weaponised gas itself was still new.