I am placing John Doreward of Rivenhall as husband of Anne Martel, proven to be mother of Walter Doreward, based on the following two entries from the UK Public Records Office online service (PRO-CAT). The first entry shows that John Doreward held Rivenhall in 1379/80, which he could have received in marriage to Anne Martel, whose family, several generations previously, is known to have held Martells Hall in Rivenhall. Anne later inherited Martells Hall in Ardleigh as well, but may have been given Rivenhall in marriage. The second entry shows that the Doreward family actually held Martells Hall in Rivenhall (not just "any old property" in Rivenhall), and that this was the Doreward family whose properties were eventually inherited by the Mortimers (namely David Mortimer and his two sons Robert & David (it is this younger David who I believe is named in the suit)). We know that Walter Doreward is of that line of descent, therefore it makes sense that John was Walter's father (and Anne's wife).
Father: Elias (Ellis) DOREWARD , of Bromley b: ABT 1322 in Great Bromley, Tendring, Essex, England
Marriage 1 Anne MARTEL , Heiress of Martells Hall b: ABT 1351 in Martells Hall, Ardleigh, Tendring, Essex, England