Learn English – What does ‘That is’ mean here

There is nothing special about the phrase "snake metaphor." The sentence that you quoted contains a metaphor about a Burmese python, which is a kind of snake. Earlier in the article, the author makes a different snake metaphor:

It took me a few minutes to wriggle into it — picture a film of a snake shedding its skin, played backward — but once I did, the women in the group were agog over my impressive physique.

In this case, the author means that he has already used a metaphor involving a snake shedding its skin, so his reference to limiting himself to one snake metaphor per column is just a joke to point this out.

Pythons are constrictor snakes (they kill their prey by squeezing them to death), so I imagine that he means that the shirt is squeezing him very hard and making it difficult for him to breathe.

In my experience, and according to web searches, "can’t bake the cake" is not idiomatic and is not common except in its literal sense of not being able to cook a cake, for example because of an oven problem.

The sentence "I don’t care how you cut it; the brother just can’t bake the cake" means that Romney "just can't get it done" and is failing to accomplish his campaign plans. This usage of "bake the cake" is found in numerous web pages that discuss planning sequential processes.

Note that Maureen Dowd’s wording, "As Michael Steele, ... eloquently summed up..." uses eloquently ironically, to indicate that the quotation is not eloquent and instead appears to be an attempt to be hip.