First, you will not be able to detumble without at least one sensor. For CubeSats the normal way to detumble is through the B-dot algorithm which measures the magnetic field change and turns on appropriate magnetorquers to slow down the motion. Since you have to measure the magnetic field to begin with, you've already established a rough attitude in two-...
I did some rough calculation.
E(Engine Force). = 800 N
m(Mass of reaction wheel) = 10 kg
r(radius of reaction wheel) = 0.2 m
w(max spin rate of wheel) = 6000rpm * 2pi / 60s
cgo(C.G offset) = 0.01m
I = m * r * r / 2 = 0.2 kgm^2
WheelCapacity = I * w = 125.6 Nms
Torque experienced during engine firing due ...
No. No object can exert a torque on itself via only internal interactions. That would violate conservation of angular momentum.
Spacecraft are constructed to have controlled, usually zero, magnetic fields to reduce torque from external fields they encounter.