To the OP, I’m not sure I would want to second-guess the engineering specifications and/or warranty instructions on any cooktop when it specifically says that it cannot be flushmounted. I do completely understand your frustration that they did not make it obvious which they clearly should have. But even if the cooktop you purchased was specified to be flushmounted, I would not do so and for some pretty straightforward reasons.

In order for flush mounting to be done properly, you first have to find someone who knows how to do it. It’s not very common to see fully flush mounted appliances and so, no surprise that there are very few people who know how to do it properly. Don’t get me wrong, I really do see the interest and attraction to the idea, and I actually don’t think it’s “technically“ impossible to get this to work, but that is in theory, and not in the field, where you have appliance installers, who need to accomplish getting your appliance, installed, fully flushed within the counter, all while not damaging the appliance or the counter.

Adding to this is that when and if your cooktop needs to be removed in order for repairs to be made, who then comes to remove it? Repair technicians, most of whom have no experience with this, may scoff at it, and refuse to work on the appliance altogether. And at this point, you now have a project that has ballooned into something a lot more complex which is the last thing you want when it involves one of the most important appliances in your kitchen for food preparation. You might continue to search for a technician who will (eventually) take on the task and at the end of the day, your appliance has been repaired (hopefully 🙄). BUT THEN, you discover that the appliance has been scratched or damaged in some way or your very, expensive countertops are now marred or chipped.

No matter how many kitchen designers promote this idea in HGTV or other Home fashion media,
I would chalk up the flush mounting of appliances as one of those things that is often marketed mostly in concept to help sell product, but is actually rarely undertaken due to the realities and complexity of actually pulling it off with the installers and technicians that are currently available to most homeowners. Maybe this will change in the future if the concept can continue to grow and takeoff, but for now, these are the risks you will run, and this includes cooktops as well as the new trend in “flush mountable” wall ovens. For those people who live in very large metropolitan areas and are willing to wait and pay for the very few specialists who can get this accomplished, I salute them. But the reality for most consumers in most geographical areas does not support making flush mounting actually happen without everything ending in tears and unnecessary $$$.