I know the changes to the uniqueness of the passive make it so they can't stack with additional copies of themselves but can you buy both of them or are you buying useless stats? Thanks in advance this is really confusing to me for some reason.
No, just like how none of the bami's cinder items stack together.
They do stack.
Yes they can, as it is a unique passive, not a named one. Bami's cinder gives Immolate, a named passive, to itself and the items that build from it. Named passives do not stack across different items, but unique ones do.
Unique passive: Don't bother getting multiple of one item, multiple items that give the same stack like 
and 
both give unique 20% cdr passives, but they still give 40% cdr togeather while two of or would only get 20%.
Named unique passive: Like the Bami's cinder line, just don't get two of the lines items. Sunfire cape and cinderhulk do not stack.
Hope this helped.
yes
They stack, they are unique to the item only.
