Okay - thanks.
I would not expect that 2 feet of pigtailed 12/2 wiring would cause the flickering problem.
That said:
First, I would check the wiring to the extended ("18 inches") circuit again. And especially all of the connections, screws, wire nuts, etc. from the junction box to the new location. All tight and with good contact.
It appears that you would be and are able to check the various outlets and switches in the entire circuit. Make sure they all have clean tight connections. (Disclaimer: If you are not comfortable with working through and checking the entire circuit then an electrician should be brought in.)
Next, I would turn off the appropriate breaker and find everything that does not work. There could be some unexpected loads on the circuit via hidden junction boxes branching off somewhere to serve other areas. Code prohibits doing so (i.e., junction boxes not being visible) but it still happens. I trust you just put a blank plate over the original outlet box vs drywalling it in......
Then look at the power consumption for each load and then total up the load. Just ensure that there is no overload or near overload. Is there a fan on the vented gas fireplace than comes on at ignition? Could draw some initial startup current.
Tedious but you could also start adding loads back one by one to see when the flickering starts...
And try things without the surge protector. Maybe it has some problem.... Unplug the TV first to see if anything else flickers when starting the gas fireplace. TV may be more sensitive than anything else to any current drops. Or use an appropriate size (wattage) lamp/light bulb to simulate the TV's load - see if that load flickers. With and without the surge protector.