For the most part, I would agree that Total Depravity is the most important point, However, Edwin Palmer stated that if any point of TULIP were wrong, then they are all wrong.
Also, P-Marlowe did not say the 'L' was THElynch pin, he said it is A lynch pin.
You're right. PM did state it was "a" lynch pin. But I'm not sure I would agree with that. There are discussions among the Calvinists between Limited and Universial Atonement. Dr. Steve posted an interesting article on the two veiws. Die hard 5-pointers would insist you must believe in Limited Atonement.
While I believe in Limited Atonement and believe the scriptures bear this out, I don't see it as a lynch pin. Whether you believe in Limited Atonement or Universial Atonement everyone would agree that the blood of Christ ONLY covers those whom He saved.