Seems clear that it was some international figure. It was a horrible way for Mathilde to die.
Was the gun sabotage simply for Jacks' safekeeping as he must of begun to suspect he might be hit - or did he not want the main target killed because he had started to thaw out and had fallen in love.
I don't think it was ever really hinted at. I'd say the gun would have been used multiple times, but it's main purpose was to get Mathilde close to Jack.
To your second point, I'd stick with him getting suspicious.
Yes probably was just his suspicion. Without the killings of the three main characters it might have made an interesting TV series.
Regarding Mathilde getting close to Jack - she could have killed him at any time during the testing of the gun - or just been a silent assassin with Jack unaware of her presence. So I'm not sure that killing Jack was her original purpose.
Of course Jack supplied the gun - but she could have got a gun from elsewhere - so did her priorities change after she met him for some reason - I guess Jack must have become a liability.
A shadowy figure arranging hits - and a hitman whose constantly having to change location.
The book has lots of detail; a lot of it regarding the countryside, but there is another hooker.
A lot of people feel she could have killed him, but he seemed very guarded to me. The only real time was when he was walking to the target, but she would need to be sure of getting him with one shot.
Maybe she was feeding info on Jack back to Pavel, so close contact was required. And don't forget, it wasn't she that ordered the making of the gun.
Yes - it's like a colour in the missing details story. I guess it's the uncertainty that makes it so watchable.
It was the opposite of a CGI fest - which is what I like.