From this appnote:
To make an AC switch with two MOSFETs, only one driver is required. Both FETs have their sources tied together, so the driver (shown as a battery on the schematic) applies a positive Vgs to both. It's a bit counter-intuitive for the bottom FET because it's upside down, but it also gets a positive Vgs just like the top one.
In the off state, both FETs have their body diodes reversed, so they block AC. In the on state, both FETs are on, so the switch resistance is 2xRdsON.
he duty cycle of the PWM signal is adjusted to control the effective resistance of the MOSFETs, thus varying the load on the UPS. I'm aware of the potential for short-circuiting, and I'm trying to to prevent it with implementing dead time in the PWM signals
This won't work: since you used two drivers (one per FET) the driver either turns the FET on or off. If one of the FETs is on but not the other, the one that's off will simply act as a diode. In this state, we have the low RdsON of the FET that is turned on in series with a diode from the FET that's off, so the switch will pass current only in one direction but it won't limit current.
If this load is intended to test a power supply... most power supplies have capacitors on the output so this will make huge current spikes and destroy the MOSFETs.
In addition to drive this arrangement you need a floating isolated supply which is referenced to the FETs' sources.
Now I wanted to make a pure sine wave dimmer for my water heater to make it use only the power produced by my photovoltaic installation without drawing from the grid, and that's pretty much the same thing as what you want to do, so here's the result:
There's a mains voltage rectifier and filter to avoid injecting HF current into mains, then a simple asynchronous buck converter driving a resistor. I went with a buck because the resistor is quite large and has some inductance, so using PWM on it directly would cause spikes and high electromagnetic field emissions. Whereas feeding it a filtered voltage from the output of a buck produces much less emissions.
So the rectified input voltage is chopped according to the PWM duty cycle, then fed to the load resistor. From the point of view of the AC IN source, the result is a load that looks like the resistor divided by PWM duty cycle with a good power factor and clean sine wave input current.
It is possible to put the diode on the negative side and the MOSFET on top, but then the driver supply needs to be floating. This can also be done by replacing the diode with another MOSFET to make the buck converter synchronous. This makes it more complicated, without much reward. The low side driven FET is simple, and using a SiC diode for D5 cuts down on diode recovery loss on FET turn-on.
This hasn't been built or tested, so caveat.