@eSPiYa Hey I'm not staff or anything but I may be able to answer a couple of your questions.
The first one depends on how you subscribed. Currently there is a "regular" and a "premium" subscription. If you subscribed for the premium member, then you receive a premium credit once a month, every 15th of the month. This credit can then be used to purchase the E-Book through this website, otherwise you are welcome to purchase the E-books through other distributors that J-Novel Club has connections to. I don't know them all off hand, but Amazon is one of them. If you would like to purchase additional premium credits, I believe the option is available now to purchase one for $6 through your account page.
In regards to your 2nd question, once you purchase the E-Book it should be available to download from your account library (assuming you buy it from here). After that, connecting it to whatever E-Book reader you're using should allow you to read it offline (I imagine). I'm not too knowledgeable in this area so you may want to confirm with someone with more experience.
Lastly, anything you purchased from this website is available for download so you can reread whenever you like. As for if you forget to download it and your subscription ends, I don't believe that makes a difference as it should already be purchased and linked to your account.
Hopefully that helps a bit with your questions.