several cranio and maxilla doctors. the problem is its brittle
Exactly. It will be more brittle than bone. Bone contains collagen and several other proteins that give it tensile flexibility and 'spring' as well as a bunch of trace elements. The crystal structure is quite interesting - it's basically a co-crystal of nano-crystals of HA and rows collagen fibrils. (Enamel is a little different in that in contains less protein and more fluorine has larger, more organised HA crystals - making it stronger, but more brittle)
As far as I know they mix the HA with organic products like blood plasma and collagen. Also I think they they use HA nanocrystals, rather than growing them with the collagen from the liquid phase Over time some of the organic elements will dissolve somewhat so I am sure that HA paste will contain less collagen than regular bone and the crystal structure will not be nearly the same. It's not called 'bone cement' for nothing....it is a very crude approximation of bone (although there is are some interesting in vitro studies that have been done on the nucleation of HA crystals and how it's directed by proteins etc)
Anyway, I digress - how long it's mouldable for will depend how its mixed although I'm surprised that A&G can mould it for as long as they claim - I'm guess they're using more organic product in their mix. I think the only way to remove it would be to file it off
If it's a small asymmetry you can a probably disguise it somewhat with some of the more dense hylauronic acid fillers as a short-term measure. In the longer term, if it's bad then try and push your surgeon to add more to the smaller side
