For the first question- a firing squad is very symbolic. In my home country, there has been talk of bringing back execution by firing squad (currently there is execution by shooting, using one shooter), which would symbolise the condemnation of the victim by the state and public. Your peers, some who may have served with you, are condemning you to death and are judging you as guilty. It encourages and is symbolic of the solidarity inside armies, so instead of one man against one man, it is a group of men, brothers in arms, standing together, who, as an army, not as a person, condemn and execute the victim. It is also a preferred option for psychological reasons, especially for soldiers in the middle of the war [1]. If one person executed the victim, that person would bear the guilt of the execution alone. If a team of people executed one person, they would share the guilt between them, could pass off direct blame for the death into others and feel solidarity and support from their peers during and after the execution. It's for similar psychological reasons that gas chambers were used in WW2, killing people is difficult- killing people who were from your country, your army, your regiment even, would be particularly hard [1]. Firing squads would have eased the guilt and difficulty of executing a fellow soldier.

Another reason is for the honour of the person being executed. To stand (or kneel) and face your death was considered, throughout history, much more honourable than a simple shot to the side or back of the head. Execution by firing squad gave those who were considered dishonourable, such as deserters, an honourable death at least, and this would partly have been to assuage the guilt of the executioners (so they did not feel they had dishonoured the person being executed) and also symbolic- whatever the executed person did, at least the army were honourable [1].

Firing squads are traditionally quite old- predating firearms [2].St Sebastian is depicted as having been shot by a firing squad's worth of arrows. It was an effective, quick death which didn't require any special equipment, just archers and arrows. People, or, more specifically, soldiers, historically were probably very desensitised in war- they didn't shoot from far distances or fire mortar shells, many saw the people they killed and watched them die. Romans would have seen gladiator fights and public executions around the world were common until just over a century ago in much of the West, and in some places, public executions are still carried out.

Firing squads were not uncommon in the English Civil War, in the 17th century, with deserters executed at Tyburn [2]. There's also records of the execution of Admiral John Byng, who was executed by a naval firing squad after a retreat from Minorca, in the mid 18th century, and in 1743, a group of people, who deserted after learning they were to be posted in the Caribbean, were executed by firing squad in 1743 [1]. Firing squads were also the military execution method of choice until 1808 in Italy, until the introduction of the guillotine [3].They have historically been popular around Europe therefore, and spread with European colonialism. Although we have records of firing squads occurring from the advent of firearms in Europe, as noted above, they did occur before, for example, in Ancient Rome [4], where arrows were used, and likely in other societies throughout history for the 'positive' reasons behind firing squads mentioned above.

Sources: [1] Public Executions: From Ancient Rome to the Present Day, [2] Beholding Violence in Medieval and Early Modern Europe, [3] Tosca's Rome: The Play and Opera in Historical Perspective, [4] Rome: A Piligrim's Guide to the Eternal City.