From experience, I think you should be ok and get the bonus although the T&Cs say that they are only for new customers so there's no way for you to complain if you don't get it.
In terms of using your in law's address and a email address, I'm not sure if this is such a good idea? I don't know how it will mess around with your credit checks against your records which would already have your address.
Previously, AMEX reps were willing to confirm in writing (through the account support tool online) that you would still get the bonus points on signup even if you had a existing card, providing that if you had a referral from a friend. This was however before they added that 'must not be a current amex customer or have had amex in the past 12 month rule' added to their T&Cs (The rule just stated 'existing AMEX cardholders werent eligible')
Plus, a referral might not be a bad idea for other cards but for the Plat charge, I think the referral bonus is only 60k on signup which is a coughpier deal than the current signup promotion on the website.
