I resorted to cons counting to confirm it for my own simple mind and... you're correct! ^_^
; bucket (5 conses)
((a . (b . nil)) . (1 . (2 . nil)) )
; "frugal" alist (4 conses)
((a . b) . ((b . 2) . nil))
Sorry I don't have time to respond to the rest of your comment right now.