Responding to your inquiries in order:
1) Log into your E-junkie Seller Admin, go to Account Preferences, paste your desired post-checkout landing page URL in the Common Thank-you Page URL field, and click Submit to save changes. All your buyers will then be redirected to that page after checkout.
2) After the buyer finishes checkout, and once the payment processor notifies us they have completed the buyer's payment, we send the buyer a thank-you email message with a link to reach their thank-you/download page in case they did not proceed there after checkout. Every download link we issue expires after the number of Attempts or Hours (whichever comes first) that you specify in each product's settings. This help page explains more about customizing your thank-you email messages:
3) The standard text on our thank-you/download pages cannot be replaced, but you can add custom text/HTML to those pages providing whatever translation or instructions you wish, in whatever language you wish. This help page explains how to customize your E-junkie-generated thank-you pages, with an example at the bottom showing what customizations would get added where in the page: