Yes, you can have a CC attached to your room so you and any people in your unit that you authorize can use their key card to make charges. We don't usually do this, however. We would rather just carry our CC with us and use it without another middle person involved. Since we have APs anyway, we usually just stick our IDs, AP, CC, and DVC card in the AP sleeve. We can either stick that in a pocket or wear it in a lanyard around the neck. I'm not into going through bag check, so I usually keep it simple.
Yes, you can use your room key to charge at the parks. You secure it with a credit card but you have the option of paying your bill with another credit card, cash or your Disney Visa Reward dollars. I like the convenience of being able to charge park purchases on my room key and then paying it off during the trip. I also like the convenience of being able to use my Disney Visa Reward dollars to pay off my bill.