Hello there,
The reason for using any kind of isolating device (opto coupler, pulse transformer, line transformer, etc.) is to ensure that the user of the equipment does not come into direct contact with the line voltage even if something somewhat typical goes wrong with the equipment (the controller). For some failure modes this is just impossible, but for the more typical isolation of the parts of the circuit that the user can come into contact with is done with some sort of isolating scheme.
The point here is that for the correct operation of the unit isolation is not mandatory, it's only for the protection of the human user. But in cases where the isolation is not going to come in the form of a device like an opto coupler, a plastic potentiometer shaft is the next best thing. This is better than a metal shaft and plastic knob, as a plastic knob comes off too easy. The plastic shaft makes it impossible for the user to touch any electrical contact inside the case, so this is the recommendation when pure electrical isolation is not possible.
The triac control circuits can be driven off of a half wave rectified DC power supply, which allows a common connection between one terminal of the triac and one terminal of the DC power supply. This is probably done already, so the main problem appears to be some sort of spike on the triac gate.
The reason for this could be because the triac gate doesnt look like it is connected properly. The triac gate (sensitive or not) should have a resistor from the gate to main terminal 1 (terminal A1 in the supplied attachment in the first post in this thread). This resistor helps prevent false triggering and would also help with spikes. A typical value would be 1k, but lower values could be required down to probably 100 ohms. Yes this then requires more current drive from the driver part of the circuit.
There is also a snubber kind of circuit often connected across the output of the triac (A1 to A2). To find out the values of this circuit refer to the triac documentation or look for other circuits on the web. This is usually just a resistor in series with a capacitor.
So the choice of how to drive the triac (isolated or not) is totally up to the designer, but since protection of the human user is also a concern some means to electrically isolate the user from the circuit is always employed. Metal shaft potentiometers must be avoided when there is no other type of isolation in place.
If there are other kinds of controller parts such as switches, they too must be of a type that will not allow the user to contact any part of the circuit even in the event that the switch 'handle' breaks off. This could mean a specially mounted switch with a secondary handle mechanism that if broken does not affect the switch or allow user contact. A rotary switch with a plastic shaft could prove effective here.
To understand what you mean when you say to connect a diode, a drawing would help here, but with the gate resistor it probably wont be necessary.