Does the oath of ẓihār take effect if a man says to his wife: "You are forbidden to me until we return to our families," intending thereby to forbid conjugal relations, and what are the implications of that?

If by not having intercourse with your wife you intended an ila' (vow of abstinence), then it is not considered an ila' according to the majority of scholars unless it is accompanied by an oath. If you abstain from intercourse with her for a period that harms her, then she has the right to refer her case to the judge for divorce, or to obtain her right to conjugal relations. Live with your wife honorably, and resolve your problems through understanding.