Thermal grease is relatively cheap. A tube can be pricey, but the amount you actually use is so small that the cost is negligible.
If he's fighting you over it, he's not a great tech guy. Don't go back there. Thermal grease (or other heat-transfer material) is vital. It is certainly cheaper than buying a new unit when the one you have overheats and dies. As to cleaning: You can do it yourself, fairly easily. Get a vacuum cleaner, suck out the bulk of the dust.
Get a soft-bristle brush and brush the dusty areas while using the vacuum to collect it and keep it from settling into the other parts of the machine.
After that, you can remove the parts and clean them individually.
Make sure that any solvent you use for cleaning is compatible with the material. Distilled water is my go-to for most things. It won't hurt electronics, as long as they are not powered, and you blow out all the water afterward and allow to dry thoroughly before reassembling.
It can be tedious, but you should only have to do it every few years or so. Be careful with using compressed air. Some parts, such as cooling vanes, can be delicate, and air under enough pressure can bend or break them. That said, sometimes, it is the only way to dislodge dust from between the vanes. It is always risky to poke through them with a toothpick or something.

To commit a crime, you have to make an intentional act. If something loads anything on your device without your knowledge, you are not culpable unless you find it and make the decision to leave it there.

Linux + Brave FTW, here! I went full linux several years ago, and never looked back. I only use it when an employer requires it. And only on *their* computers. The learning curve can be steep, depending on how much control you want, and what you want to do with it, but it has been well worth it. Only problem these last couple days has been that Brave seems very slow. Not sure why. At first, it was faster than firefox, but now... Hmm.