Delta Sigma DAC-like switching of a mains AC load
You are way over-thinking this. Given the long thermal time-constant of the system, there's no need to worry about high-order modulators, or how the sample rate of the sensors relates to the sample rate of the output.

Yes, delta-sigma is a good way to control the heater power, but a basic first-order implementation will be more than enough for your purposes. One side comment: it would be friendier for your power company to switch based on whole cycles, rather than half-cycles.

Your control loop produces an updated power setting at a 0.959 Hz rate. Let's assume that this setting is an integer between 0 and 255, representing 0 to 100% power.

You want to update the triac gate at a 60 Hz rate. Let's assume you have an interrupt that's synchronized to the rising zero crossings of the power line.

Set up your interrupt handler so that it has an 8-bit accumulator. On each interrupt, add the power-setting value to that accumulator, and if there's a carry out from the addition, turn on the triac for that cycle.

It really is that simple. The overall average duty cycle of the heater will match the power setting exactly. If you really feel you need higher resolution, you can use 16-bit values for the power setting variable and the accumulator in the ISR.