I highly doubt Dae Ho was faking being a marines for multiple reasons -
1. Why would the actor go so in-depth about how marines act with a close friend if Dae Ho was just faking. Kang Ha Neul talked to a close friend who was a marine and made Dae Ho's personality based on that friend, even his hairstyle;
2. Since his introduction, he is seem flinching/hiding himself: He flinched when hearing gunshots, which is obvious; He tries to hide himself when Young-Il was yelling at himself; He flinched when 044 yelled back in Mingle; He was flinching when Young-Mi was asking for her Unnie. Which means, he is a traumatized man, principally anxious towards loud noises;
3. IRL Story Wise, there was a Shooting Incident in 2011 in South Korea Marine Corps, same period Dae Ho was in the marines, if they were to put in the series storyline;
4. At why he was dismissive during his talk with Jung Bae and changed subject, He was FORCED by his father. It is obvious he is not in a good relationship with his father, who possibly physically abused him, and even his mother, who (based on a specific line) isolated Dae Ho from other kids. So, He was forced into the marines, which is already a bad thing, and considering his comments about his past, Dae Ho is not the "conventional man", that means, he was probably bullied or even abused in the marines due to toxic masculinity;
5. As to why he was terrible at using the gun, it can have different reasons: Marines doesn't use a MP5, they use a K2 assault Rifle; Due to his trauma, he was scared of using it; There is a chance that he was Discharged due to Mental Health issues (Such as his Anxiety and PTSD) before he could even learn how to use guns.
I'm a Dae Ho defender and that's all.