I dabbled with Facebook Instant Game a while back. for the test app portion it is optional, but you have to basically make sure your app is properly linked to the Facebook SDK and change the app ID to the ID the Facebook Developer website gave you.
Before I discuss the 2nd Area below here is a helpful link on it with examples https://developers.facebook.com/docs/games/instant-games/guides/in-app-purchases/
As for the 2nd area “In-app Purchase Integration” it is just asking for an explanation of the steps in YOUR game to access them. i.e "Step 1 load the app by (however your game is hosted most likely it is hosted on facebook’s game section) step 2 click the shop icon at the bottom right of the screen. step 3 click buy (virtual currency) Gold,Crystals or whatever the name of your VC is. step 4 the In-app Purchase API should load.
Test User the 3rd category means they will use one of your testers client i think to test the game for themselves. i.e Admin Account is you and you have 3 test users Bob,Tom and Dan. if you enter the user Tom into this field the client that Tom had they will use to test the app.
“Please provide Facebook Login Integration notes that are at least 20 characters.”
I found Info about your issue with this here at step 4. https://developers.facebook.com/docs/facebook-login/review/how-to-submit/
" In addition to adding notes for each permission request, complete the following submission form fields:
Facebook Login Integration
Please provide general instructions of what your app does and step-by-step instructions that show how to access Facebook Login.
If you are submitting features that don’t require Login Review, you will not see the Facebook Login Integration field and you do not need to fill it out.
Upload Your App - For iOS/Android only
iOS : Upload a simulator build file or provide a Apple Store ID. Help: How to create a Simulator Build.
Android : Upload a APK file or provide a Android Package Name."
So tell them how facebook login works in your app, does the API launch on first use? Is there a button that handles login? Tell them about it in that field.
Some of what i said may be wrong but, Hope i helped you some.