I'm sorry that I can't help you with any of the technical questions, but I just wanted to say that he sounds EXACTLY like my son. Not deaf (turned towards noise), great at following body language, no apparent receptive language (he knew his name, that was about it), did not mimic, did not have any words.
At his 15 month appointment, my ped recommended that I call EI and that we get his hearing tested. Just about EVERYONE I knew told me "ridiculous! He's fine. Some kids, especially boys, are late talkers."
Because it took so long to get the hearing test, he had his EI evaluation first. The results, in the words of my pediatrician, were "sparkling." All except in the communication section, where he was not even CLOSE to the line below which he was eligible for services. I don't have the numbers right in front of me, but it was something like 100 was the median, below 85 he would qualify... and he scored something like a 60.
According to his hearing test, he definitely wasn't deaf, though he did have poor hearing. He heard most of the beeps. But a few appointments with the ENT showed that he had severe fluid buildup. Because he was so young (the ENT, our speech therapist, and the social worker liason with EI have all told me individually that usually peds don't notice this problem until the 18 month appointment at the earliest, and that I'm very lucky to have such an on-the-ball one), they made his case a priority and he had ear tubes within the week.
That was in the last week of August, and the changes over the past month and a half have been really astounding. He has about 10 words now. His receptive language has really exploded: he seems to understand just about everything (within reason, obviously). And the interesting side effect is that he's really taken off on walking! He only took his first steps in mid-August, and I guess that with all that fluid in his ears his balance was really bad. He was a really late sitter too.
He has a speech therapist who comes once a week, but after the initial period it seems really unlikely to me that we'll need her. Her job is mainly to help us catch up on his receptive language. He's not actually totally out of the norm for talking, but he missed a pretty valuable window of receptive language learning. Luckily, he seems to be catching up quickly.
So that's our story, and it had a very happy ending. The ear tube surgery was a little scary for me going into it... my poor little baby put to sleep and laid out on the operating table

But the surgery itself took all of 10 minutes (seriously), and he woke up a bit grumpy but within a few hours was back to his normal self. He also discovered popsicles while in the hospital room, and that if he cried the nurses would keep bringing them to him! So I think he actually had a pretty good time of it, all things considered

Hopefully your situation is similar to ours, and the resolution just as simple. It's great that you noticed it so early.