That stinks about the auto seeds. I would give them a shout and see if you can get some comp seeds.
And I'm just an old outdoor guy who almost never uses lights. But many who do say that at least one hour of dark a day will help the plants live a sort of normal life. The sleep time is when the toxins and such is removed.
When do you start shortening the day, and how much shorter each week? Assuming 14-15 hours Length of Day in mid May, you will need to trim 9-10 hours in 9-10 weeks. Chop off too much time all at once, and they might early flower on you.