Nic,

1. whether user will be collecting (receiving) or loosing credits for actions can be set within the settings of our plugin. If you set a negative number ( ex: -10), users will lose 10 credits. If you set this to positive number (ex: 10), users will receive 10 credits.

1. not all activities are connected to "User Credits" plugin.
First of all our plugin is connected only to actions that deal with adding content.
User credits plugin doesn't work with actions related to viewing the content.

The list of actions  that deal with adding content depends on plugins you have installed on your site. Available actions is visible within settings of our plugin.

If payment gateway is absent and you have set User Credits plugin in a way that users loose credits for actions, at some point of time, your users will have a 0 balance and won't be able to purchase credit packs since you don't have an active gateway. Make sure to set our plugin so that your users have an option to receive credits as well.