

Customs union and single market aren’t the same thing. Customs union means no tariffs, and a unified tariff policy on those outside the union. Nothing more, nothing less. There are active discussions about doing this.
Single market is joining the EU, with regulatory alignment and free movement, as you quite rightly say.
Kill switches are almost entirely a misconception, as far as I understand it. What’s far more dangerous with US defence products is reliance on them for spare parts, repairs and upgrades. Piss off the US in some way, your very very expensive aircraft no longer gets spare parts, plane can’t fly safely, it’s functionally useless.