In life, there is nothing that is 100%, but the scenario you describe is pretty close to 100% NOT to happen.
Having said that, let me relay an event that I am familiar with:
As part of our aircrew training, we had to do roughly a week of Aeromedical Training (AMT). This consisted of briefings on Hypoxia, Spatial disorientation etc. These briefings were followed up with practical demonstrations. For Spatial Disorientation, we would sit down in the world's most evil barber's chair. We would then close our eyes and turn our head sideways and lay it flat on our arms in front of us. The instructor would then spin the chair until you told him it was no longer spinning. (In reality, what had happened was that the fluids in your inner ear were now spinning at the same rate as the chair was so you no longer felt the motion.) The instructor then suddenly stopped the chair and you were asked to return to your seat. Your eyes were twitching, but because your head was turned to the side, one eye was twitching up and down and the other eye was twitching left and right. The fluids in your inner ear were spinning but in one ear they were spinning clockwise and in the other they were spinning counter-clockwise. Let's just say that after a step or two, even the toughest in the group needed help back to their seat and most needed to use the "extra" garbage can that was in the front of the classroom.
None of that, has anything to do with DCS, I just wanted to provide a little context. To study Hypoxia, we did two "chamber rides". On the first profile, we would put on our masks and breathe 100% O2 for 30 minutes to help off gas the N2 in our body. We would then do a "climb" (where they reduced the pressure in the chamber) to a simulated altitude of 25,000 ft. In pairs (buddy system), one of us would remove our mask (while our buddy kept theirs on) and we would do a simple test until we noticed something unusual. That was our hypoxia symptom (it is different for everybody and it can be totally random). Once we noticed our hypoxia symptom, we would plug back in to 100% O2 and then we would watch as our buddy did the same test. When everybody was done, the AMT Techs would repressurize the chamber and take it back to "ground level". The second profile was to simulate an ejection at 30,000 ft. The chamber was depressurized from ground level to a simulated altitude of 30,000 ft in less than half a second. The chamber fogged up, there was an incredibly loud bang and sinuses cleared rather dramatically. After everybody indicated that they were OK, the chamber was repressurized back to ground level and we were done for the day.
That evening, the AMT Techs stopped by our quarters to check on us to see if anybody had a DCS hit. They checked us again the next day. We were all cleared to fly home (commercial) the next day, a full 24-36 hours after being in the chamber. On the flight from Winnipeg to Toronto on the way home, one of our group (my boss) got bent. His hit was so bad that Air Canada had to divert the plane to the nearest suitable airport where he was met by an air ambulance which flew him at "tree top level" to the nearest chamber. He did a typical DCS Chamber run and has been fine ever since.
Just because you wait the 18 hours does not give you a 100% guaranty that you will not get bent on the flight home even if the airplane does not depressurize. My boss had about 30 hours between his (admittedly aggressive) chamber rides and his commercial flight.
I would say that the odds of what you describe are infinitesimally small, however, sometimes it is nice to take an extra day and sight see or to sit out the last day of diving if you have concerns.
