when you are in "launching soon" mode can you still edit your launch items?

You absolutely can.

Great question! I’m also curious about how far in advance you should create the "launching soon"

you can edit anything about your launch up until the very moment it's launched :)

Yep, you can continue to edit your launch page details until the moment you hit the big launch button. So keep tweaking those screenshots and copy until it's just right! 👍