I'm glad this was helpful!
As for your first question, you can talk with Tapas to set the first (x) number of episodes free, and then after that, it's all locked. There is no set amount that needs to be free (but the first few always are). There are generally no free episodes in the middle, since it'd be unfair for the readers who bought it.
As for key prices, I'm not sure overall but I was able to set the key prices (although Tapas has a better understanding of around how many coins per episodes is a good amount).
And, yes you can! I only have my one novel up on Tapas right now, but if I were to make a new series, I would be able to have a free series as well!
Let me know if you have any more questions!