There's no worries on the safety aspect, I'm a very 'sensible' person and as such ensure everything I do is safe and doesn't cause me any (unwanted) pain.
As for the spare key, the plan will be to place it on a peice of paper that cannot be copied (like my Mistresses handwriting, or a snippit from a magazine (I wouldn't know where it came from), and then sellotaped around completely. That way the only way to get the key is to tear into it, with no way of putting it back as it was.

I just don't know how I'm doing to handle no masturbation or contact on my cock for a month, although I'm interested what it will be like...