TEMA:

Is there a way to create forms where a user can log in, start the form and then come back later and edit the form to change/complete it?

Hi,

if this is breezingforms form, then you can publish this form as menu, and in menu's settings->Add form->enable Submission editable by user. After that when user logged it into your page, they can edit form (user must be logged in otherwise he will not be able for edit).

About login in form, by default you can find login in form in joomla, please check documentation:
www.joomla51.com/tutorials/joomla-basics...-a-joomla-login-page

Regards,
Tihana