"I had never heard of the name Bradley Manning before it was published in the press"
I can well believe this, but it is not enough. If he ever sent a message saying something like:
"Mr. Anonymous contributor #1234, I believe you should be able to copy this classified info on a CD-ROM that you can disguise as a music CD of Lady Gaga, then put on earphones to pretend you are listening to it", then he is in trouble because he can be charged for conspiracy.
If on the other hand, he only ever wrote: "thanks for this information you want to send to us, I will set up a server for you", then he might be all right.
As for the SMS messages, I can believe it is a set up, but I can also believe he is embellishing more innocuous messages. After all, he has already exaggerated somewhat the 'crimes' uncovered by the cables.