I got the warranty info directly from WD's site and spec sheets. RPM is NOT the primary factor in determining seek time, that only affects rotational latency, which is one of at least 4 components of access time, the other three being track seek time, head settling time, and head select time. Seek time is generally the largest of those, rotational latency second largest, and the others are minor by comparison.
Amount of ECC is not only dependent upon 512/4k (AF) drive, that's one factor, but most "enterprise" drives from most manufacturers have greater ECC and most use lower track densities to allow faster positioning (faster seek). For instance, compare the data sheets for the 7200RPM desktop and Enterprise (Constellation ES) drives from Seagate. Note the "enhanced error correction" and better "non-recoverable read error" rates (which are directly related to ECC recoverablity) on the ES (enterprise) drive, and that's comparing a 512b sector ES drive to a 4K/AF desktop drive.
As I said, you analysis was generally good, you just missed a the 3 items I noted.