As sparks pointed out some geysers are not easy to access whereas others may not be a problem at all.
As it is the norm for plumbers to fit geysers in such a way that the electrician cannot access the terminals without without experiencing extreme discomfort
I would personally design the controller to be installed in either configuration as per the schematics I gave above. Maybe supply just a controller and supply separately two installation kits for it, that way you're more likely to cover all bases.

On the installations where layout necessitates a layout where COC is required then obviously you'll need to subcontract an IE to issue one. If the installation can be performed as per the second schematic then a COC wouldn't be required and it would be a bonus.

As far as COC costs go I would approach two or three local registered electricians and put together a proposal where maybe they give you a preferential rate according to volume of sales or according to an exclusivity agreement where you would guarantee to only use them for your work. There would probably be 'spin-off' work as well for the electrician because, as you've already stated, many geysers are the victims of sub-standard installation practices and would require remedial work such as earthing straps and isolators/spreg to be installed.