1) As of right now you have to override our CSS on your page. Our CSS stylesheet is inserted after the first <script> tag, so as long as you have your CSS after the first <script> tag in index.html, all of our stuff should get overridden.
Chrome's dev tools/firebug will be your best friend for this, but the structure for achievements is:
(if there's an icon for the achievement you'll have these as well)
2) The payment API being integrated into the plugin is on the todo list. I've integrated "Show all achievements" and updated the plugin with it
3) Encryption isn't possible right now. I'm not familiar enough with Construct 2 to know if you can create a backend for your game. So long as you have a backend, you'll be able to. Perhaps there's a way to get it working well with the AJAX plugin, I'll have to take a look
4) Fixed (no need to update the plugin for this fix to work)