Before you go to the trouble of replacing things, are the keys in good condition?... not worn? If they're all worn to some degree, you might try a new key. A dealership can make a new key given the VIN. I did that a while back for my Avy. Cost wasn't too bad; around $10.
Sorry, I can't answer your questions regarding the immobilizer.
1992 Camry LE, V6 (3VZ-FE), ABS brakes, dark emerald pearl, owned since new. Replaced HGs @332k, now at 357k
1996 Avalon XLS, ABS brakes, super white II, acquired w/ 139k, now at 296k
2001 Yamaha FZ1, Ivan's jet kit, resprung, Ohlins rear shock, Race Tech cartridge emulators in forks, 48k miles