yes you are correct -if you failed to do a decent dil flush as per the checklist order and bring the po2 back down and jumped back on you might see .7 and think you've turned on the 02 -in fact you could probably do a pre breath and jump in the water and it might still read .65 but it would be RED (warning ) so a hot drop might pick it up to .7 again and you might think its all ok -once again demonstrating the checklist is in a certain order for a reasonThe prior steps, for the dil, would ensure the loop ppO2 was pulled down below 0.7. So, failing to turn the O2 on means the subsequent steps will tell you that O2 is off - if you are paying attention at all.
Also, just for the record, with rMS the pre-breathe generally takes 1 - 2 minutes. If you don't have rMS, then the prescribed pre-breathe is 5 minutes. So, with or without rMS, you should catch it if you fail to turn on the O2. Assuming you are putting any effort at all into following the pre-jump checklist.
I did some tests at home - starting at .21 with 02 turned off you've got about 17 breaths and approx 1min 27sec before your light headed (.16) if you miss that your computer is flashing RED for that long and your solenoid is working away and you dont notice it before your temp stick has warmed up then i dont know what else can be done to warn you