I know what you mean about Chewie. Salvatore did a great job sending the ol' Wook out in the ultimate blaze of glory.
It all depends on how George worded the orders, though. If he said "Luke, Leia, and Han can't die", then that's that. If he said "Luke, Leia, and Han can't be killed", then MAYBE they can loophole it to make them die of natural causes.
Pretty sure it was the latter, because it would be kinda weird for them to go through time and reaching like 900+ years old and still be around whooping Empire/Sith ***.