I have recently given one of my dead Thralls signus septimus, Storm thrall as a scroll to hold,later in the next battle he ran off but came back with a storm thrall.After searching his inventory he used it up after play testing this I found a few rules:

1)Entering new cells(loading screens) Diables AI for Thrall's thralleaing Signus's thrall wil just stay where was before loading screen when it comes back

2)He will only use scroll it if combat is initiated and enemy attack you

3)Thralls and followers don't play well together

4)Dead Thralls will spam any other scroll you give them(dread zombie,Blizzard,conjure atronachs,Dead Thrall)

As for companions I test out the same thing with derkeethus same as above but he thrall wil die or dissapear when entering loading screen but normal summons will go threw

Sorry if it wordy I will and more later when I get more info