Late here but on demand spacing. I think they plan on a 2 minute spacing for the first 4 corrals followed by a 3 minute for the next 4-8 then up to 7 for the latter corrals. I believe that one will that there is a variability even in that spacing depending on crowding at the 1.2 mile point (one lane bridge.)
I think we as runners will eventually like not knowing the wave start times as it provides more flexibility on race day to the race director to keep the first choke point clear.