1: No, the plastics currently used for 3D printing would not handle the immense vacuum for prolonged periods. Also there was good reason why RCA abandoned metal bodied CRT's.
2: No, switching from one gun to another would also cause the beam alignment to change slightly. If I'm doing this in my head correctly at 7AM, this would require tuned electronic compensation via the deflection yoke.
Also a triple gun arrangement is FAR bigger than the neck of most black and white tubes.
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