The Ignition method is best used for just gathering and exchanging data, not for live, direct control. One other note on this, this would only allow for communications to Rockwell PLCs. This does not allow for direct access to say VFDs or remote I/O.
You are correct in that Codesys is the best option for this, as they have an ethernet/ip scanner module.
This method would allow you to connect to PLCs and devices such as VFDs and remote I/O.
Last to your TLDR, you can use the Rockwell Modbus TCP AOI(First link in search). It can be used as both a client and server. Then you can use the Opto 22 Modbus Integration toolkit for PAC Control to get your data from the Rockwell PLC into the EPIC.