A) When we perform a discount, this is generally a percentage that depends on several factors.
As you could already read in our JMS 1.2.x product detail, it is already partially compatibilie with Joomla 1.6. We just know that they are continuing to refactor plenty of things and we can not predict if that will have consequences to create a JMS 1.3.x or just limited fixes.
C) It is not in the intention of JMS to create a similar system to VirtueMart to manage a shopping cart with the ordering, confiruamtion and payments. This is the reason why we redirect to this extension.
Directly manage the subscription require to manage the order with their status and plenty of other things that is out of our scope.
If you want to create a bridge with other system, you can do it. This is documented in JMS user manual chapter 4.