Pesachim 39: Wherefore Horseradish?!

Maror! 5 vegetables that are listed as effective to fulfilling one's obligation of maror. And what kind of preparation (or lack thereof) leaves them still fit for the obligation. And what kind of status is permitted. What ARE these 5 vegetables? And why horseradish, since it's not on the list and the Gemara is very clear on details about this list that make it clear that it doesn't include horseradish (including Rabbi Dr. Ari Zivotovsky's article on the same question). Also: Why lettuce? And why doesn't the Gemara discuss bitterness?! Including some of the rabbinic interplay in seeking out the best maror. Plus: A new mishnah on some additional possibilities of chametz (is this out of order?) for non-edibles (animal fodder, a shower scrub, a poultice).