One of the reasons for the Signatories to the various treaties codifying war crimes. The treatment of POWs, Perfidy, and who qualifies for protections of The Law and Customs of War are found in Geneva III treaties.
First, Perfidy is a war crime, and thus, those that surrender in order to lull opposing forces into a trap are guilty of a war crime.
Where it is unclear is "how far up the chain of command did the orders to commit Perfidy get sent.
In your second case the grounds are a bit fuzzy. First, Geneva III list some requirements that combatants must meet in order to qualify for POW protections. These restrictions are as follows:
(a) That of being commanded by a person responsible for his subordinates;
(b) That of having a fixed distinctive sign recognizable at a distance [a uniform or something that allows identification];
(c) That of carrying arms openly;
(d) That of conducting their operations in accordance with the laws and customs of war.
By committing Pefidy, the 2nd ARB Unit 1 lost their immunity, but even then, the surrender might not have been legit, even if it wasn't a trap.
The reason for this is while the white flag is a protected symbol and is a war crime to misuse, and the orders "No Quarter" cannot be issued, the Geneva conventions are rather mum on the procedure for which a lawful surrender may be performed beyond a ban on perfidy. You can even surrender lawfully without waving the white flag in a literal fashion. With the language lacking, most countries do not look to closely at the nature of a particular surrender. Generally, most countries make no obligation to honor a surrender at the very last minute. In the case of Unit 2, as they put up some decent resistance before they waved the white flag and given Unit 1's previous behavior, the 1st Chasseurs are well within their right to not accept the surrender. Surrender is generally not honored if it's done to save your ass after loosing an engagement (you can not inflict the most damage you can and take no consequences for it by surrendering. If you are going to surrender, do so prior to the last minute.).
The flaw in the premise is that in Order for "No Quarter" to be violated, the standing orders of 1st Chasseurs has to be "Even if you are capable of taking POWs under your present conditions, you may not take POWs." Nothing indicates that under standing orders, 1st Chasseurs was told "take no prisoners", as they were more than willing to do so with Unit 1 the previous day. As they were already engaged in a fire fight with Unit 2, who surrendered only after it became clear they were not going to win the skirmish, combined with Unit 1's prior actions, would be more than enough to justify their refusal to accept the surrender in this moment of time.
At time of writing, there needs to be more specifics about the state of the engagement between 1st Chasseurs and Unit 2 to make a determination if they had a legit lull in fighting and show of sincerity that would have made conditions optimal to accept the surrender of Unit 2. But part of the unspoken enforcement of the Laws and Customs of Warfare is that if you or your comrades in arms violate the terms of the Laws and Customs, you should not be surprised if the opposing forces stop giving you the benefit of the doubt (Because war crimes tribunals are held after the war ends... and are typically prosecuted by the winning side against the actions of the losing side. 1st Chasseurs being prosecuted at all is very dependent on if their side loses the war.).
Edit: The use of the phrase "The Laws and Customs of Warfare" was used because, contrary to popular belief, the Geneva Conventions are not the only treaties that define war crimes, just the one that comes up the most, as it deals with matters related to protections afforded to Combatants and Non-Combatants and who qualifies as one of those two categories. The full body of treaties has treaties that have been enforce in 1675 and as recent as 1997. Most conventions are related to bans on certain types of weapons on the battlefield.
