I spoke over shabbos to the talmid chochom agricultural expert. He is actually quite a bit more than an expert, having been the head of research for the Israeli Agricultural Institute for years.If you argue with him over his field of expertise, he will turn you into horseradish.
I asked him about horseradish leaves and whether they are biter or sharp. He laughed and said that the same chemical that makes the horesradish sharp makes the leaves sharp. Not bitter, sorry. Not marror, sorry.
As far as doing “what our ancestors did for decades” , which is quite a small slice of historical pizza in itself, a little bit of knowledge can reveal alot of tuth. As Reb Doniel mentioned, insisting on practices which are clearly wrong make the system into a chuchah u’telula.
Bottom line. Horseradish isn’t marror, and one should not attempt to fulfill the mitzvah of marror with it. Use romaine lettuce. Ok, be MACHMIR and use romaine lettuce if it makes you feel good being machmir. Use the horseradish for gefilte fish.