Does Apple provide any insurance to protect/reimburse the customer if a Genius misuses their access to the customer's device?
Yes people do give service professionals access to there homes, but unless they are hopelessly naive they don't leave credit cards, money, jewellery and guns around.
If you read the fine print in the agreements you make to have such items you explicitly agree not to allow other people access to them.
The scary thing is that responses here don't seem to be playing the devils advocate and actually seem to be believing this is OK.
Ultimately, it is an issue of trust; and I argue that the relationship of trust with Apple started the moment you decided to buy an iPhone, and continues to apply when you ask Apple for support.
Trust is not binary or one-dimensional. You have no choice[1] but to place some hope in them working in alignment with your interests when the build the product, but it doesn't mean you should be careless about retaining some control in the process when doing business with their retail staff. Their staff is human and heterogenous, and you know there are more levels of QA in the phone production process than in the customer service, and customer service employees have much lower risks of getting caught & much less to lose career-wise than iOS engineers if they violate your privacy.
People complain about Facebook and Windows 10 too. "You're implicitly trusting Facebook/Microsoft anyway" is not really true there either, you're just tolerating their privacy behaviour in exchange for value, and hoping to escpe harm.
[1] If you want a smartphone, that is (the competition is worse)