Is there a way to complete a task and automatically accept the next task in the process, without having to pass through the worklist?

You can automatically asign a task, but tasks are intended for human interaction.

http://techcommunity.softwareag.com/ecosystem/documentation/webmethods/wmsuite8_ga/My_webMethods_and_Task_Engine/8-0-SP1_Task_Engine_Users_Guide.pdf

You need then to click on “Accept” manually, or modify the task (then is accepted and you jump the step to accept it), for wM 7 and 8.

wM 8 provides you the option to open a task from a task type inbox.