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.
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View my fuel log 2015 Mirage DE 1.2 manual: 42.7 mpg (US) ... 18.1 km/L ... 5.5 L/100 km ... 51.2 mpg (Imp)