Well these aren't new cabinets but I am feeling like hiring an installer would be the right choice. I certainly didn't sign up for cutting into my cabinetry.

"The micro fits the space just fine but the trim kit needs the opening to be larger so its clips can be attached."

Larger than what? Page 6 of the installation instructions shows a cabinet opening whose inside width is 28-1/2" and inside height is 17-3/8" +- 5/64". Is your cabinet opening a different size?

Yeah once I got the old one out I found the opening was only 26x16. If I can find an installer he'll have to cut the opening larger now. Really there should be a way to sink these brackets into the cabinet though.

"If I can find an installer he'll have to cut the opening larger now."

That isn't going to work. According to the website specs oven alone is 26-3/4" wide -- You can't even set the oven in the cabinet without the trim kit. As shown in the installation instructions, the trim kit attaches to the sides of the oven and increases its width to (slightly less than) 28-1/2". If the cabinet is only 26" wide, you can't cut it down by 2-1/2" ( = 28-1/2 - 26); you'd destroy the cabinet.

And what about the height? Is the opening of the height required by the trim kit?

The micro is only 24" wide but with the trim airflow redirect on it, it is a tight fit for the height. The existing cabinet has 3" of extra front face (on each side of the opening) that could be cut away without destroying the cabinet (assuming there is a good tool to do that in place).

I would just leave the microwave in the cabinet with no trim. Microwaves don't last long these days, and when the one you have now (which replaced your previous one) breaks, you will again be faced with a new MW that is a slightly different size with slightly different trim. Just leave it without the trim, and be free of this problem.

"The micro is only 24" wide"

So the NN-SD975S website specs which show the oven width as 26-3/4" are incorrect?

The "face front" doesn't matter. What matters is the width of the oven cabinet. You wrote that the inside width of the cabinet is 26". Assuming that the cabinet is made of 3/4" wood, then outside width of the cabinet is 27-1/2" ( = 26 + 3/4 + 3/4) which is less than the 28-1/2" required to fit the trim kit.

If I’m reading this, and the link correctly…this is a tabletop model being made to look built-in, with a trim kit added to it…and, at an additional cost.

We faced a similar problem, when doing our kitchen 7 years ago. We decided to forgo the expensive trim kit, and just put the mw on the shelf…to which our contractor made a small adjustment. The way it fits, it nearly looks built in.

There’s just enough air space for heat to escape.

Well I found a contractor through home depot's referral service. He will be doing the cutting tomorrow. @wdccruise, the 'inside measurement' for the cabinet is about 32" but the current opening is only 26. He's going to open it up to match the spec required for the trim kit. If this looked more like a finished shelf, I'd consider your idea @darbuka but the old unit was held up by 2 2x4's and the inside are looks quite unfinished. Hopefully the contractor can open it up (without just ruining things - no more dominos falling please) and I can put in the micro and trim kit.

"the 'inside measurement' for the cabinet is about 32" but the current opening is only 26"

Well, I hope you've communicated more accurately with the contractor than you have with us.