The other foods served at the meal can be eaten later. Eating the third meal on this Shabbos is difficult to do, since the third meal is supposed to be eaten after midday. At that time, we may no longer eat chametz, matzah or egg matzah. After that time, one is required to minimize his intake of food so as not to ruin his appetite for the seder. There are different customs concerning when exactly the chametz is sold this year, since many people eat chametz and use chametz dishes until Shabbos morning.
All times are for Cleveland Heights, Ohio.
This custom is only l'chatchillah. If the chametz was not burned by this time, it may be burned anytime prior to the onset of Shabbos. For the sake of the children, who are a primary focus of the seder, the seder should begin as promptly as possible once Shabbos is over. The horseradish should be ground and stored in an airtight container until the seder. When feasible, even the seder table should be set on Friday.