How to embed plugin configuration form elements into another form?
I have solved this by making sure my forn had a unique element names to avoid collision with the Action's configuration. I have added a 'x_' prefix to names of all my elements. Since my form did not use #tree globally I have put the Action's configuration under a 'configruation' key and in form validation and submit callbacks I have provided this array as $form['configuration'] to the validation and submit handlers of the Action plugin instead of the whole $form array. So far it works withotu any issues.