is there any article or write up that lists or discusses the tasks in FamilySearch that are NOT an option when using HELPER mode??
Yes, see https://www.familysearch.org/en/help/helpcenter/article/what-does-a-helper-do
I noticed that when using the FamilyTree App on a smart phone - and helping someone (helper mode) - - I could not do the "Famous Relatives Activity" on behalf of the person I was helping (rather it would force me to logon with my own credentials)
but when I do the same thing on a computer - it does allow me to do "Famous Relatives" acitivity in helper mode.
Is this a bug?
Not really a bug, but rather a limitation. There are a few things in the mobile Family Tree app that simply take you to the website -- they aren't really app features. The Family History Activities are in that category. So when you are using the Family History Activities, everyting is done in your browser on your smartphone, not in the app itself. So for those activities that require a session, you have to sign in again to the website on your browser. And if you are in helper mode, you have to additionally sign in as a helper.
gotcha - - not very user friendly in that way - but I understand what you are saying - that when you access it via the app - you are actually leaving the app to go to Famous Relatives (in the browser) and its not sharing credentials - so it forces you to log on again - and go thru helper mode AGAIN.