On enhancers, strands, and zygosity
...an enhancer sits on just one of DNA's two strands (usually the same strand as the protein-coding DNA gene itself).


Yes and no. Enhancers are in fact sometimes palindromic, so they're mirrored on both strands of the DNA.


  This is unlike a protein-coding DNA gene, which might need to be on both DNA strands


No. Protein-coding DNA is always on one strand only because transcription doesn't suddenly switch strands.


  in a so-called homozygous state, in order to surface as a phenotype - like the classic case of blue eyes.


No. Homozygous means on both chromsomes and has nothing to do with strands or the need for an allele to be homozygous to produce its phenotype because it's recessive.


  And this is [an] evolutionary advantage: an organism doesn't have to wait for a change on both DNA strands. The bottom line is that evolutionary tinkering is in principle much easier with enhancers...


This is completely all over the place and makes no sense to me. Either "strands" is used in the correct way, in which the previous statement is wrong, or "strand" is used as "chromosome", in which case there is no reason why an enhancer should only be on one chromosome. Just like the gene, it can be hetero- or homozygous. The statement about "having to wait for a change on both DNA strands" is more relevant to whether something is dominant or negative. Regardless, I don't think it's possible to make any statements here about enhancers allowing for evolution to progress faster.