Modifying electrical boxes with field mounting and knockout provisions
As long as the box is rigidly secured, you're good to go regarding the box mounting arrangement

Considering that the concrete wall can be considered to be a structural member, not an exposed building surface, and that the box is mounted independently of the finished building surface, NEC 314.23(B)(1) applies here, which permits the field-drilled mounting hole and screw arrangement used provided it secures the box sufficiently and the AHJ is OK with it upon inspection:


  (B) Structural Mounting. An enclosure supported from a
  structural member or from grade shall be rigidly supported
  either directly or by using a metal, polymeric, or wood brace.
  
  (1) Nails and Screws. Nails and screws, where used as a fastening means, shall secure boxes by using brackets on the outside
  of the enclosure, or by using mounting holes in the back or in
  a single side of the enclosure, or they shall pass through the
  interior within 6 mm (1∕4 in.) of the back or ends of the enclosure. Screws shall not be permitted to pass through the box
  unless exposed threads in the box are protected using
  approved means to avoid abrasion of conductor insulation.
  Mounting holes made in the field shall be approved.


Field KOs in a box should be OK as well

Making knockouts (whether of standard or nonstandard size) in electrical boxes in the field is a long-standing practice in the electrical industry (in fact, some enclosures ship without knockouts, expecting the user to make their own).  As far as I can tell, as long as the cable is adequately secured to the box with a standard cable clamp (as per 314.17(C)), it makes no difference whether the KO is factory or field made.

However, a box that shallow doesn't have enough fill for...much of anything

The shallowest Carlon nonmetallic boxes (at 1.25" deep), while technically deep enough for a receptacle (at only 0.69" for the receptacle + the minimum 1/4" of extra depth required by Code), do not provide enough box fill at 8in3 to accommodate a hot, a neutral, a ground, an allowance for a cable clamp, and the double allowance for the receptacle yoke -- for 14AWG wire, this would require 12in3 of fill.  As a result, you'll need to slap a 5/8" deep non-metallic single gang box extender (or construct one using a 1/4" and a 3/8" stacked atop each other) on there to gain that extra couple of cubic inches of fill.