Not OP, but studied forensics in college.

• What did the guy think? Probably trying to do something special, maybe he couldn't decide which gun to use? Maybe he liked them the same.
  

Mostlikely, he wanted to make sure he died. Also, a lot of times when a suicide is extremely messy, it's on purpose. They want their family (or SO) to find them and for them to be traumatized and feel guilty.

• Did he think about forensics, who will find the mess? Maybe it was a special for them.
  

Their thoughts are elsewhere. They usually don't think about forensics, albeit if they do its either very briefly or in cases where they do, they sometimes try to make the scene look like a homicide.

• Did forensics boss find this little exercise funny? I guess they were not under pressure of other work if they had time for such activities.
  

Actually reconstruction is taken very seriously. Reconstructing things like the skull is vital in a lot of cases. They may find a suicide/homicide victim and have no idea who the person is (no ID, no fingerprints on file, no DNA matches. There next best option is asking the public, but they need some sort or picture to show. The best way to develop this skill is via real world experience. So even if they know who the victim is, reconstruction provides valuable experience.

• How did they manage to glue his head back together? Serious question.

No 100% on this, but for bones, they use super glue.

• How many people were involved in the reconstruction? What did they think about it? (fucked up/gross/awesome/standard-sarcastic-attitude/ all of them)

It varies. Sometimes it just 1 person, other times it may be multiple people, and sometimes multiple people will reconstruct different parts (skull, hair, skin, etc...)
Most people take their job very seriously and treat the deceased with a lot of respect.