How to achieve Shannon's capacity if coding introduces a spectral efficiency loss?
You cannot convert the Shannon Capacity formula to a BER "waterfall curve", because the capacity theorem states that in the AWGN channel, there exists a modulation and coding scheme such that the probability of error is arbitrarily small with a capacity bound given by $B \log (1 + SNR)$. Sadly, it doesn't tell us how to construct such a modulation/coding scheme that approaches this bound.

For a code length to approach infinity means that the code depends on an arbitrary number of previous symbols. Heuristically speaking, the "optimal" code will take advantage of correlations in the errors across symbols, so the longer the code is, the more it can exploit this redundancy.