Is it okay to eat the marror and chazeret from the keara (seder plate) ?
Yes, it is preferable to eat the food from the ka'ara. That's actually why we put the food there- not just to remind us, but to actually be used.
The exceptions are the zeroa (roasted shankbone or other meat) and the egg, which come to remind us about the karbon pesach and chagigah (the festive offering which was eaten first, before the karbon pesach) respectively.
The egg can be eaten at the seder itself. It's even proper to do so, in place of the korban chagigah which was eaten at night.
The zeroah should not be eaten leil seder, since the minhag is not to eat roasted meat (so we shouldn't mix it up with an actual korban pesach.) The Mishna berura brings the chayei adam's ruling that the zeroah should be eaten during the day meal.
You can read about this in English in the sefer "Halachos of Pesach" by Rav Ribiat pg. 638 question 59.