My outlined response would follow these steps:
1.) A moral dilemma in which a character is set up to fail in some point
(at least when one uses a particular scale of a code which allows several
different scales) is a classic scenario. Tolkien discusses at length one
such scenario in his essay on the mediaeval poem, Sir Gawain and the Green
Knight. (And he also discusses the idea of a code with several scales of
varying degrees of severity.)
2.) I don't think it necessarily undercuts Frodo's self-sacrifice if he
is brought into such a dilemma.
3.) I agree that Frodo has Christ-like qualities, but we should never
forget that he is by no means Christ -- Frodo of necessity must sin in some
point.
This deserves a much longer response, but I haven't the energy anymore.
By the way, I like St. Anselm
too.
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All that is gold does not glitter,
Not all those who wander are lost;
The old that is strong does not wither,
Deep roots are not reached by the frost.
From the ashes a fire shall be woken,
A light from the shadows shall spring;
Renewed shall be blade that was broken,
The crownless again shall be king.
