The slight discount you may be seeing is because your TA is booking the race registration directly through RunDisney/Walt Disney Travel Company. When you get the race registration online, there's an extra fee associated with the website. I've seen charities sometimes have discounted prices but other than those, that would be the only explanation I have!
I do not unfortunately. I call RunDisney about once a week to see if they have any new information. I plan to give them a call this week to check up again. If the screenshots were correct and Marathon Weekend will go on sale on July 20th, we should be hearing more information very soon. What should happen, or has in the past, is that after the race is announced officially TA's are given an allotment of bibs to book from. The allotment will sell out, be restocked, etc until the allotment for TA's officially sells out when no one has anyway of knowing. I've had times where the races have sold out quickly, and others where I was able to get my clients bibs about a week after it opened to the public.