According to Intel, your CPU runs at 2.2 GHz: https://ark.intel.com/content/www/us/en/ark/products/134906/intel-core-i7-8750h-processor-9m-cache-up-to-4-10-ghz.html, and CAN turbo up to 4.1 GHz. This does not mean that it will run that fast all the time, only when voltage/thermal environments allow. You indicated that your system is thermal throttling once it hits 95C, which sounds about right. The question I have, is when you say it throttles down, what speed is it throttling too? If it is going down to 2.2GHz, that isn't really thermal throttling issue, it just means that the voltage/thermal environment is not allowing it to turbo up. If it is going below 2.2GHz, you have a problem. It sounds like Alienware has already replaced the motherboard which would rule out an actual hardware issue. The only other controllable piece is the environment you are using your laptop in. Is it setting on a hard, flat surface, or are you holding it in your lap or other soft material? You should really use it on a hard flat surface, to allow the cooling system to work at its best. What is the ambient temperature in the room? Ideally you probably don't want to use it in a room that you wouldn't be comfortable sitting in. Also, I assume you are using it plugged in? If you aren't, then the system (by design) will throttle down to preserve battery life, even when pushing (like playing a game). You can combat this, by puttign Windows in "Desktop" power mode, but this still won't prevent the system throttling down to 2.2 GHz, as this is the default state. On battery, you can expect to see it throttle down to 1 Ghz, or even less depending on what you are doing. Again this throttling isn't a thermal issue, but rather a battery saving feature.
FWIW, I use to game on an older Alienware M17 R3, with a Skylake (6700QH) that was rated for 2.6 GHz, with turbo up to 3.1 GHz, but it almost never went to 3.1 GHz. It spent most of the time at 2.6 GHz. This is the reason I switched to a desktop when I upgraded. My 8700K runs all day at 4.4 GHz, so it was a huge upgrade from 2.6 GHz :-).