The Cambridge Grammar of the English Language by H&P says (Page 1394):
(d) Infinitival extensions
 i a. A few replies are still to come. b. There are still a few replies to come.
ii a. One letter is (for you) to sign. b. There’s one letter (for you) to sign.
In [i] a few replies is understood as subject of the infinitival, while in [ii] one letter is understood as object of the infinitival – see Ch. 14, §6.3.
CGEL analyzes that [ia] and [ib] mean the same thing, and that [iia] and [iib] mean the same thing. But isn't one letter (for you) to sign in [iib] normally analyzed as an NP (especially when for you is omitted)?
Assuming that there are two possible analyses of [iib], one according to CGEL and the other treating one letter (for you) to sign as an NP, is [iib] inherently ambiguous?
In general, is the existential construction inherently ambiguous when accompanied by an added component such as a non-finite VP, an adjective phrase, or a preposition phrase at the end of the construction?