Hi Dave, Is it nearly 4 years old already, it only seems like a year or so ago
Yes, simply glue the cabanes in position. If you wish to add re-enforcement, keep it minimal to keep the weight down. Also, should something untoward happen its best the cabanes break free without causing further structural damage to the fuselage.
Again, a balsa firewall will be just fine. If in doubt laminate with an off-cut of 1mm ply however anything more is overkill and again adding unnecessary weight.
Keep everything as light as possible and it'll fly like a peach - start adding the ounces and you'll have a mini pylon racer on your hands