As long as no mirror around, yes.
It depends on the person. Narcissists can be highly trustworthy. Narcissism is being self obsessed and loving yourself too much to where you think you are perfect. Someone could be highly narcissistic and if you tell them a secret, they wouldn't care enough to tell anyone else because their secrets are better and your secrets take time away from paying attention to themselves. Also, a narcissist is unlikely to borrow money from you or some other situation where trust is required in the first place. They prefer 1) their own company, and 2) the company of people who will praise, adore, and dote on them. Narcissists are more concerned with your loyalty and your trustworthiness than their own. My guess is that they are mostly trustworthy since they don't want to lose any "fans" and care much of their reputation. But also, they will "use" people. Everyone else exists to cater to them and worship them, in their mind. Simply put, they don't care enough about anyone but themselves to even bother telling their secrets or not paying them back as they wouldn't even borrow money in the first place.
If the person posessed additional negative traits BESIDES narcissism, yes untrustworthy. But if they were so narcissistic that they had no room for other negative traits, they are probably trustworthy since they don't often have the oppurtunity to not be.
Yes. Just because they love themselves does not presupose they are untrustworthy.
mmmm. narcissist will be willing to go over you at any cost to get what they need. I don't know how you can trust that, you can trust they will step over you when needed.