Yeah, they usually work if you get the location before the counter offer. But never worked for me when i got couter-offer first.
Should only be a few weeks till the fix
+rep
Shooting the counter offer works, I've used it quite a lot. Doesn't matter if the NPC has started making the offer, if you hit him before he finishes, (normally 3 messages), he will shut up and the mission will still work.
If you are in an anarchy system there is no problem with this. If not you will get a bounty. You don't have to kill him so hit him once and it will only be 200-300 credits. He will start attacking you shortly after you shoot him though, so don't hang about.
Remember to pay off the bounty asap or the cops will be on to you when you are scanned.
+rep
And just to add some additional pointers based on my experience...
1. Don't take more than 2-3 missions at once. I stocked up on 9 missions one time, and that was quite a headache. While the NPCs who tell you were to find the target only seem to show up in the designated systems, the NPC mission forkers will show up anywhere... had one try to fork my mission over 160LY away from the designated system once. The more missions you have active, the more likely that the USS you check will have a mission forker in it instead of your target.
2. Just because the Galaxy Map says "Anarchy" doesn't mean that you won't get a bounty for shooting the mission forker... I've found it safer to use non-populated "None" systems instead, although like the poster above says you can just pay off your bounty after locating the target. In those situations, I like to pick systems that have an Outpost.
3. Wanted to reiterate that once you've had the NPC who tells you where to find your target, you don't need to shoot the messenger who tries to fork your mission... the mission will display the alternate in the Transaction tab, but just keep checking the USSs in the designated system, and you'll find your guy.
4. If you have multiple targets in the same system, there's a chance that killing one will give you credit for the others too. This can be a good thing if you just want the reward or a bad thing if you enjoy fighting the anaconda and their potential wingmates.
5. If you pop into a USS looking for the first NPC to tell you where to find your target, and the NPC is talking to you but everything they say is blank, I recommend shooting them to prevent them from forking your mission. Don't know if a "targetting" NPC ever talks in blanks, as I don't let them finish...