I can believe the problems opening EPCOT, though I don’t believe everything was down at once. The rides were too state of the art and untested against an immovable opening date.
SSE did have RV locking issues. A cast member was positioned under the track in 180top to manually check the rotation was locked in place for the descent, and manually swing the cars if needed.
However, Space Mountain was never meant to open in 1973. The original opening date was June 1974 which was missed by a mile (the date was set before construction began) but by that date the project was only 80% complete. By Christmas 1973 there was no ride to ride. Bill Watkins was the first human to ride on July 1 1974, and by October 1974 testing was well underway with the project 85% complete.
Parts of the track were rebuilt during the first half of 1976 to smooth out troublesome sections, one side at a time.