Hello,
1 - Yes, HikaSubscription allows you to create several plans. Since the system is based on HikaShop products for the sales, you can make the listing you want to let the customer select what he wants.
2 - HikaSubscription do handle upgrades but with the extension of the duration, not with a discount.
Afterwards, you can see to have different products which can extend more or less the duration of the subscription with the upgrade. There are still possibilities but the component do not interact (yet) with the HikaShop product prices.
3 - The upgrade do an upgrade of the subscription. So the subscription remains but is for another plan.
By definition, a customer can have several subscriptions but thanks to the plugin/event system (or with ACL and product limitation), you can forbidden the customer to buy other subscriptions.
4 - There is no such kind of feature but because it is based on HikaShop products, you can use the HikaShop feature to extend the price. Like "product option" or "custom field" + custom plugin.
Afterwards, for the creation of the administrators, it would require custom development.
Regards,