あれで is equivalent to あの様でいて, whose literal translation is "(That thing) exists in that way (and/but)".
Thus it can be used as
which can be shortened to:
and then further to:
What exactly is shortened needs to be guessed, so I can't be 100% sure what is implied, but one interpretation here is that the way the gorilla exists (appears) already resembles Dudley, and if it had blond hair on top of that it would resemble him even more. Another valid example interpretation would be
あの様でいて、さらに金髪だったらもっと好きになるのに (which, however, seems less likely given the context!).
The "despite appearance" usage can be explained as follows.
Something like this:
can be shortened to:
and this further to:
which implies, despite this person looking confident, they are actually anxious (again, what exactly is implied by the last sentence needs to be guessed).