It's all good, I got them done mate
No it is just one layer of plastic to drill through, the painted part of the bumper.
The dimensions I mentioned in step1, are the safest regarding the car. A technician from an authorised ford service center recommended them. But make sure they comply with the manufacturer of the sensors. Some recommend lower drilling height so you won't lose low obstacles. This is both for safety reasons as well as for functional. If you install them lower than the manufacturer suggests, they might give false posisive detection every time you reverse. And I am sure you don't want that
For some reason I can't put a pic up but have a look in the link in my bio thing (just below )