A Synod is not a decision-making body. It offers advice to the Pope, who makes any real decisions. This is from a reply to “frequently asked questions” prepared by the USCCB:
What role does the Pope play in the Synod?
The Holy Father convokes the Synod and establishes the topic to be discussed. He ratifies the election of members of the Synod chosen by the various Episcopal Conferences and appoints additional members. He determines the agenda and presides over its sessions, either in person or through others. The Holy Father receives the proposals of the Synod and acts on them as he deems appropriate.
There is no possibility that a group of liberals could hijack a synod and change Church policies regarding married clergy, women priests, etc… The Pope convokes a synod, picks its topic, determines the agenda, presides over its sessions and decides whether to act on the bishops’ suggestions – or not. I’m not sure you understand just how tightly controlled the Church is by the Pope and Curia and just how few liberal bishops there are after the long tenure of John Paul II, who appointed most of the world’s bishops.