There is a screw on the side.
Look at the part # to see if it is orig . [ more likely not]
The reason for changing the plug/coil connectors before proceeding with further test is that as soon as the ecu recieves an ignition skip, it cuts the elect sig to the injectors... so the stumble is most noted as low rpms [ as you state] .
When it does mis-fire , it also effects 2 plugs rather than one as this is waste spark system and 2 plugs fire in series.
Codes on other modules will be your next step, but you want to assure ignition first without spending big $$$ . The connectors are known faults for you problem, and changing them is easier than scoping them out..