a) Tilting the body downwards will ward off dust from sensor but will expose more dust to lens.

b) Malaysia overall has humidity of around 80-85%. No need toilet.

Not sure whether more worrisome or same. At least sensor got self-cleaning unit but lenses don't. I try to keep the body and lens horizontally while changing lens. I'll always put on the lens cover once removed and only open when want to put back in.

You'd better worry more about static charge than dust when in countries with low humidity. It'll fry your equipment. Furthermore, the countries over there have cleaner air than here.