2. The antenna does screw out of the base, so that shouldn't be an issue.
3. I've used the foam pads and straps with a canoe on a pickup truck, they work very well and are easy to put on and take off. I don't think it's worth investing in roof racks unless you know you'll be using them often.
Unfortunately I've never used that setup on the Mirage, but I don't see why it wouldn't work. The roof might cave a bit (I can hear it pop just going through the air dryer at the car wash) but hopefully the weight distribution over the foam pads would prevent that.
View my fuel log 2015 Mirage DE 1.2 manual: 43.6 mpg (US) ... 18.5 km/L ... 5.4 L/100 km ... 52.4 mpg (Imp)