Once again back to the topic at hand, the goal is to build a particular hearth pad that specifically meets the code requirements of the particular stove the OP has purchased and about to install.
The use of thinset as a bonding agent has been long established and provided the thinset (or any suitable adhesive) will bond to both the plywood and the cement board will determine if it gets used in preference to or in addition to any other method of securing the cement board to the plywood.
Please note the use of the weasel word suitable in the above statement.
The OP is in the process of obtaining the information required to meet code, the pad will get built to meet code.
Every different pellet stove can have different requirements, several Harman folks have found that out (along with a few dealers) after the installation.
Over building sometimes has huge benefits.
The use of thinset as a bonding agent has been long established and provided the thinset (or any suitable adhesive) will bond to both the plywood and the cement board will determine if it gets used in preference to or in addition to any other method of securing the cement board to the plywood.
Please note the use of the weasel word suitable in the above statement.
The OP is in the process of obtaining the information required to meet code, the pad will get built to meet code.
Every different pellet stove can have different requirements, several Harman folks have found that out (along with a few dealers) after the installation.
Over building sometimes has huge benefits.