Foolishly I downloaded "Chapters of Deceit" last weekend without checking forums about the achievement problems.
Sadly, I too was only able to have 3 of the 8 DLC achievements unlock.
I went round and round with EA customer support and they're official statement was that once they turn content over to Microsoft (software or DLC) it is Microsoft who is getting the money/points for the download and so it is their call on refunds.
Then it was round 2 with Microsoft (I was on the phone with them for over an hour) and at first they tried telling me that it's EA's problem...they're responsible for any content published for their games.
I jumped through all their hoops, deleting and re-downloading the DLC...deleting and recovering my gamertag, clearing the cache...everything. Finally just for my own amusement I tried playing through the DLC with my brother's silver account...he hadn't even played Army of Two, let alone earned the mulitplayer achievements and the DLC achievements started popping up for him.
When I explained it to Microsoft's customer service, their supervisor and finally an escalated response rep...they seem to think that the problem lies within the achievement system which they said they ARE responsible for.
That was Tuesday night 7-27-10 and my demand for a refund of my microsoft points was escalated...I did receive a call from someone last night 7-29-10 but it was basically to say the issue has been escalated and that they could call me every 48 hours to provide an update on my case/claims status....I'll post something when I get a resolution.
I'm thinking about filing a claim with the Better Business Bureau, but I'm not sure what that would accomplish...maybe a class action lawsuit is the next step if Microsoft/Sony and EA won't take ownership of this problem and address it accordingly.