Subscriptions and subscription-lines

Did I understand the idea behind subscription correctly:
The subscription represents a contract, defining e.g. payment terms, whereas the subscription lines define the service (which might change over the life-time of the contract).

Related: Is there a simplified view for subscriptions which will always hold a single line. (Example: 1 week test subscription for a newspaper.)

Mainly yes it is what is described in the documentation: Sale Subscription Module — trytond_sale_subscription latest documentation

No.