Paul (Kawmeister) has had alot of experience with this. I have a standing offer to all members that when it comes time to swear your Statutory Declaration, I will take your oath for free.
I have only obtained lost registration once before (Ebay Bobber) and am currently in the same process you have described. I have sent the first Statutory Declaration to ICBC with a $7 cheque and am awaiting a response.
I was surprised to read about ICBC refusing to give you the details. I am not sure what is up with that. I'd like to see what you sent them. From the points you have described, the only thing I can think of is that your letter did not establish a prima facie right of ownership.
A couple of years ago we had a guest speaker at one of our meetings from Bartons. She was a font of information and most of us go to Bartons when there is an issue. I am embarrassed to admit that I have forgotten her name. She no longer does the ICBC stuff, but the person she trained is just as good.
I have read many Statutory Declarations. Basically you tell the story with as much detail as you can muster as to how you came to "own" the bike. Paul has a theory that including the magic phrase "I have no knowledge of this motorcycle ever having been registered in British Columbia before" is a key factor. I'm not sure why but he swears by it.
Your question comes up frequently. I frequently see motorcycles for sale without papers. If my current endeavor pans out easy enough, I may add this to list of special skills.