What is the maximum wall thickness this kit be used for?

I have just looked up the details of this product on Amazon, and it gives the tube length as 350mm (approx 13.25in). The cowling which goes outside the property has a 100mm spigot, so I would say, providing your wall is not more than 13.5 inches thick, you should be OK, but all this information is available on the product page.

Will a 100mm hole be ok or must it be 110mm?

The internal diameter of the wall sleeve tube is 100mm and external diameter is 103.2mm. The screw holes on the cowl are placed on a diameter so 150mm so a 110mm hole sounds suitable.

How does the outside cowl come apart so that it can be screwed to wall??

There there are six round nodules located on the hood that locate into holes in the back plate. These allow the hood to be pulled apart from the back plate. The back plate can then be fixed into position and the hood can then be pushed back on and the nodules will relocate and clip into the sockets.