Untangling the smokes
What happened is that a /3 cable got run from the smoke detectors to the switch box, but with the red wire capped in the switch box as the light switch should not be connected to the interconnect wire. Then later on, when the 3-way was disconnected, the homeowner was wondering what to do with the "extra" red wire, and hooked it up to the switch as "all the wires should be hooked up", in his mind at least.
However, that's not how it works. With the breaker off, take that red wire out of the switch and cap it off with a wirenut once again (you may need to tape the nut on to get it to hold). Now, you won't have to worry about any issues that could be caused with your smoke detectors by the light-switching arrangement.
What you'll need to do is run a switch spur from the existing switch to the new switch
Once you have the situation with the smoke detectors sorted, run your /3 cable between the old and new switch boxes. The ground wire in the new cable ties to a grounding screw in the old box (since it appears to use screws on a backstrap for grounding instead of the normal scheme) and to the grounding arrangement in the new box, while the black wire from the existing /3 cable nuts to the white wire (taped black) in the new /3 cable instead of being run into the switch, and the red and black wires in the new /3 cable connect to the brass screws (use the screws please!) on the existing 3-way switch. While you're here, move the black wire from the /2 cable from the backstab to the black screw terminal on the switch.
In the new switch box, tape the white wire black and connect it to the black screw on the new 3-way switch. The red and black wires then land on the brass screws of the new 3-way. Once you're done with both switch boxes, then you can button everything up, turn the breaker back on, and enjoy!