Where when and for what reason did the Minhag not to eat anything from the Kaare (Seder Plate) which is practiced by Americans that are many generations Americans?
It is written in the Shulchan Aruch (או"ח תעו: א) writes that there are those whose custom is not to eat roasted things on the seder night so that people should not say they are partaking from the Pessach sacrifice, and there those whose custom permits to eating roasted things on the seder night. However, the later commentaries say that the prevalent custom today is not to east to roasted things, both according to the Askenazi and Sefardi custom. Because of this there is another custom of not eating the roasted "zro'a "- shank bone from the seder plate on the night of the seder.( Mikra'ei kodesh pg. 485 note 10). The Shulchan Aruch (או"ח תעג:ד) also says that the seder plate should be placed only before the person leading the seder and all the participants should partake from his seder plate. Furthermore, there are those whose custom is the maror which specifically came from the seder plate even if they complete their "kza'it" from elsewhere. (See Seder Ha'ruch pg. 509, Mikra'ei kodesh pg. 394). Needless to say the "charoset" is on the seder plate and each participant has their maror dipped into it. From all the above we can infer that other than the shank bone there is no reason not to eat from the seder plate. Therefore, unless there is some specific custom from a particular community there should be no reason not to eat from the seder plate.