What is the computational power of Parsing Expression Grammars?
$\#$ here is just a symbol; it has no special significance. (It's often used in the construction of an modified language to indicate a new symbol which is not in the alphabet of the original language.) 

$\#^{g(x)}$ is, therefore, the unary encoding of $g(x)$; in that sense, it is a count.

The double-triangle used in the later proof is just another such symbol without special significance.

In response to your second question, the paper seems to be pretty clearly state (on page 4) that the question of whether there is a CFL with no PEG is still open. In particular, it is not known whether the language of even-length palindromes has a PEG, although it is certainly a CFL.