This is a pretty drastic change from the original!
PRing this to you so that you can get some of the credit too!

Mostly I am:

• Making main-output and preview-output mostly consistent.
• There was no need to pass around char *name/groups; it can all get loaded from the config during init.
• Refactored the code to have init manage all of the start/stop/reset logic.
• Added handling remote stop/start signal events to update the config that would update the plugin Settings UI when it is next shown; The current code does not do anything fancy to dynamically update any currently showing Settings UI.

Old code == inconsistent:
main-output.h:

void main_output_init(const char *default_name, const char *default_groups);
void main_output_start(const char *output_name, const char *output_groups);
void main_output_stop();
bool main_output_is_running();

preview-output.h:

void preview_output_init(const char *default_name, const char *default_groups);
void preview_output_start(const char *output_name, const char *output_groups);
void preview_output_stop();
void preview_output_deinit();
bool preview_output_is_enabled();

New code == consistent:
main-output.h:

void main_output_deinit();
void main_output_init();

preview-output.h:

void preview_output_deinit();
void preview_output_init();