How to convert USB 2 device with USB A connector to flippable USB C
Well, turns out only the data connections have to be hooked on both A and B sides, and only one CC is needed (A5 or B5, does not matter since the connector is reversable):

Kind of makes sense since in USB2 configuration, CC2 (B5) becomes VCONN:

Tested on:

Google Pixel 1
Google Pixel 3
Google Pixel Slate
ASUS Chromebook C434
PC (with adapter)