Nerado

Just to follow up on this, I've included screenshots so you can get an idea of what it will look like after you submit your application and it becomes available on your account.
 
I can confirm that after you apply online and your case becomes visible on your account... you CAN see what attachments made it as well as attach additional documents. HOWEVER, you should definitely be sure to attach everything you can up front when you first submit you N400. They may not recognize or accept attachments later on. So I think that is a very important point to remember.

Once you sign up to the myuscis website you can see what you have already submitted as documents in the documents section, and you can at any  time add some more documents.

You can log into the myUSCIS website to see that the attachments are there.  It is also possible to upload additional attachments, so you could add anything which did not upload properly.

I don't think that you'll get the chance to submit additional documents after submission, unless with an RFE. The application form will be closed. You'll get: 1) a snapshot pdf with the list of submitted evidence on the last page; and maybe 2) an additional responses pdf if you have to write in explanations.
 
If you're anxious, why not take a screenshot of every step that you made? As long as you know that you attached the documents and your account shows that they have been attached to the submitted application, there is nothing more that you can do. At least you know that you didn't do anything wrong.
 
imo I would take the chance to submit it during that session when you're completely ready, and with the application completed and attached documents.
 
Good luck!