having sHBD allows you to get 15% APR based on a specific time range you decide right?

It's locked up until it's unlocked which takes 3.5 days to go from being staked to unstaked. Keep in mind it's still transferable while being locked up, it just cannot be withdrawn outside of VSC while being sHBD.

if the whole idea is having a shorter time window to wait, means I could use that sHBD at anytime?

sHBD is transferable, it just cannot be used outside of VSC without waiting. Regular HBD on VSC can be instantly withdrawn from VSC to a regular Hive account at any moment

will be available options to change the sHBD for example, for sETH?

This is part of our future plan to build cross chain wrapping/bridging technology. It would likely be regular ETH, but yes that's the plan in the long term future.