comparative - Can you ever drop the first ‘as’ in “as . . . as” constructions?

Yes, in most cases, notably when the "as" used interchangeably with "like" in a simile.

For example:

"He is as spry as a mouse."

"He is spry as a mouse."

"He is spry like a mouse."

I would use the first "as" if possible, because it lends a clearer meaning to the sentence. Also, the longer the the first part is, the more it makes sense as you speak it.