1) Stripe plugin: allows you to configure "one to many" payments?
CBSubs Stripe does not support Stripe Connect. So no it does not support one to many payments.
2) Donation: this option is described in the CBSubs book, is it still available in the current versions?
Yes, Donation plan types are available.
If yes, is it possible to plan an individual donation plan for a specific beneficiary (an author for example)?
No.
) Merchandise: is still planned in the current versions?
Yes, Merchandise plan types are available. Their name can be misleading though. CBSubs does not support physical sales. It's entirely digital. Merchandise purchases are basically one-off subscriptions.
4) Is it possible to provide for the payment of additional fees during the payment process?
Yes, you can increase the price based off various criteria using CBSubs Promotions and a negative promotion. For tax there is also CBSubs Tax. You will not be able to increase the price based off payment method however.