Hi,
Will soldering on an additional 12” mess with the calibration?
What calibration..?

Length is no problem, in our converter/inverter units the wires to the current transformer could exceed easily 3 meters or more.
It´s like Dave already said, the current transformers output is a current that is flowing through the wires until it will be converted into a voltage inside the powermeter via a internal shunt resistor.
So the length is not a problem.
Should the CT wires be twisted or untwisted or does it matter?
Normally not, except the wires are lying directly beneath (switched)power leads.
Any suggestions on a housing so I can mount the panel meter to a wall?
According to your link the power meter got a snap-in housing.
Just buy a slightly bigger housing and cut a suitable hole in it, make two drill holes on the bottom to screw it on the wall before "snap-in" the power meter.