Originally Posted by RichardKopf

The answer you received in point 2 is incorrect. The 500 nights do not need to be from platinum years. From the T&C: Lifetime:
1.What are the requirements to become SPG Lifetime™ Gold?
To earn SPG Lifetime Gold status, an SPG member must have a minimum of five years of elite status during his/her SPG tenure AND 250 eligible nights since joining the program.
2.What are the requirements to become SPG Lifetime™ Platinum?
To earn SPG Lifetime Platinum status, an SPG member must have a minimum of 10 years of Platinum status during his/her SPG tenure AND 500 eligible nights since joining the program.