Possiblity of missed undisplaced talus neck fx union without surgery. Missed for 3 month. No avn found after 12 week confirmed by bone scan and MRI?

Yes. Yes, you can confirm avn after 12 weeks with bone scan or MRI or ct scan or even xrays (usually at 6-8 weeks and the sign is called hawkin's sign). Hope that helps!
Yes . If it is undisplaced it can go on to union but you would need to limit wt bearing and activity / no smoking as smolers always heal less well/ optimize vitd level / and maybe ultrasound stimulator called an exogen unit fractures are usually best seen on ct scan and this is the best way to follow the progression to union / these are difficult problems / best of luck.