The problem isn't the forecasts, which are accurate enough to be backed up by the observations.
Densities across all levels and across the entire red area may be at question, but nobody is saying that everywhere from 0-FL200 across the entire red zone has a fixed ash density, and nobody could ever say that. Some places will have, some places won't. Its in the same way that fronts don't have solid bands of identically intense rain, or showery airmasses aren't solid CB. So lets strike that one off now. The aviation industry has decades of experience of the products provided to know how they work, so pleading ignorance because they're volcanic ash products rather than sig wx, cat / turb, etc is odd given they've never complained about those.
I'd also imagine the VAAC are providing charts and information based on guidelines or regulations from ICAO. I'd imagine that the local regulators are also working on the basis of ICAO guidance, as well as information from the engine and airframe manufacturers. I'd imagine that the airlines are taking information from across the board! So perhaps more questions need to be aimed in that direction? The answers might be readily available, but everyone appears to be looking in the wrong place.
The biggest pressures here are commercial pressures. Therefore the issues are between the airlines and the regulators / manufacturers.
And saying all of this is based on speculation? No - its based on OBSERVED EVIDENCE. Its based on real ash falling from the sky, its based on satellite measurements, LIDAR, etc etc. Now whether that observed ash is safe to fly in is not for the VAAC to say.