I recently achieved the platinum in mercenaries 2 and it wasn't half as bad as I thought it would be after reading all the horror stories about the HVT captures.
The reason why it was so easy for me is the German cut version of this game. Back in the day, when mercenaries 2 was released, the German age rating for games was rather strict when it came to violence in video games. We often had reduced or completely removed gore effects in games, although they were already rated 18+.
However, in mercenaries 2 they didn't cut the gore effects (there are barely any), but the HVT extractions.
Yes that's right: In the German version of this game you DONT need to extract the HVTs. It's enough to walk up to them, melee them and put handcuffs on them and they will instantly despawn and count as captured alive. That pretty much makes the most difficult and annoying trophy piss easy, since you don't have to worry about the HVTs getting killed during the extraction process.
I assume this has been cut because it looks like the player is taking a hostage and stuff like that wasn't allowed here back then. It's actually kind of funny, that this cut turns out to be a favor for the player.
Regarding the ONLINE: Me and my friends were only able to connect to our second PS3s in our homenetworks. So you will probably need two consoles and two copies of the game.
Also, if you decide to abuse the German version and play trough the campaign to unlock the 20 different airstrikes, you will need two German copies, since they can't connect to the international versions because of that cut.
The race and the airstrike trophy should pop for both accounts. For the airstrikes simply play through the story to unlock them and call in each of the 20 once in a coop match. The turret trophy seems to be glitched in a way that it only works for the host, at least from our experience.
EDIT: I forgot to mention a glitch in the game, which can still screw you over when going after HVTs. It might happen at some point, that the game counts an HVT twice. If that happens immediately reload your save (you should always manually save before each HVT) because otherwise your stats will say 10/10 HVTs for a faction, but there will still be one on the map that didn't count. So always pay attention to the messages on your screen, when you capture an HVT.
Oh and some of them like to blew themselves up with a rocket launcher, before you can reach them. So yeah, just pay attention to actually capture each of them and reload your save if necessary and you shouldn't have any problems with that trophy.
EDIT2: Another important thing is the game language. Although it's the German version, the game should still be in English, if your console is set to English. So you don't need to learn German for this