Browsing cost == 0 and always will.
Sigmoid curve will allow very small nodes to get rewarded but encourage use up to average required storage. Otherwise we have all tiny vaults and that will be of no use to anyone. So sigmoid allows even small vaults to be rewarded but also encourages greater provision. A balance of reward verses requirement.
Most likely coded in, but several ways to achieve this, very low on our priority list at the moment to finalise it. All tokens will go to a single account, then all transfers will be done and account deleted. We can for instance make this read only for all so there is a view of it, but as I say it’s almost a zero considerations at the moment. Getting launched now is a 100% commitment for us in house and we are thinking of little else 
After launch I can happily spend days/weeks on each element of the network but I did learn last year of the cost to development of this so a mix of really good documents, more formal proofs and then have the convo’s as almost nothing is independent and there are effects of every single algorithm.