Not all of the Siastock has been sold. If you look at this page: http://www.siacoin.com/sianotes.html, you will see the following statement:

"One Sianote can be directly converted to 0.00039% of host income from renting storage, or 10,000 for 3.9% of all host income from storage. Currently, there are approximately 1250 sianotes in circulation, or just under 0.5%."

For the near future, no more Sianotes will be released into circulation. While there are 10,000 Sianotes total, there is no guarantee that the final host income fee will be 3.9%. We are committed to having a fee of at least 0.5%, because this is a fee that we have promised to our investors. We will probably have a fee that is higher, because then we have some income for the company (which will give us power as a corporate entity to hire developers, market our product, purchase security audits, and potentially many other things that require a strong financial base).

We made 10,000 Sianotes to give ourselves flexibility to release more in the future if we decided without needing to create a new asset on the AE. We have no commitments to put more Sianotes in circulation than are already in circulation.

I think this covers everything you asked.