Yes!
You can shop at Costco without a membership if a member buys a gift card for you.
No, but if you asked, they would probably let you in to look around, you just couldn't buy anything. If you know someone with a membership, you could give them money to buy you a Costco gift card, which you can use to get in and buy without a membership.
As far as I know there isn't a guest pass but maybe you could call ahead and ask for a tour? If they won't do that you can go with someone you know who has a membership and look around.
Thanks! Your answer is awaiting moderation.