Keys---We didn't have this issue, and neither did any of our guests. Maybe it is something new? We were married in November. I know you can request a new key with a signature, so if all else fails, I guess you could do that.
Tipping---I agree with Lori, it did get rather annoying, so be prepared for this. This was one of the only things that rubbed me the wrong way about this resort. In my opinion, the service should be 100% all of the time because it is all inclusive. But you will find that you will only get awesome service if you tip. We experimented with this on several occasions, and sadly, money will get you the best service, drinks, food, etc.
Type of Money---Either of those options will work. However, keep in mind that your bank/credit card company will charge a fee for using the card internationally. Also, make sure you notify them of your travel so that they don't close down your account for suspicious activity. And keep in mind that the law limits amount of US cash per transaction to $100.