I will try to unpack its one point at a time and demonstrate to you why you are wrong.
1. According to the Gospel writers, Christ died around 3pm Friday afternoon.
2. The Jews measured their 24 hour days from 6am onward instead of 12am the way we do. Their days are no less 24 hours for this differentiation.
3. On Saturday afternoon at 3pm, Jesus would have been dead for 24 hours.
4. On THE THIRD DAY ( Friday day 1, Saturday, day 2, Sunday day 3) Jesus appeared to Mary early that morning so we know it was sometime after 6am Sunday. Now count the hours from 3pm Friday to 6am Sunday and tell me what you get.
5. When I said Jesus was dead for three days I meant that Christ rose on the third day. I assumed the reader would get my point. Whether it was one day or two days or 10 days, the point remains. JESUS WAS DEAD.
6. You're assertion that they were terrible at math is based on a strawman. The gospel writers wrote that Christ rose on the third day because it was on Sunday that He rose. Friday, Saturday, and Sunday.....Sunday is day three.
I will have to respond to the rest of your post after I get off work I apologize.
However, you made the claim that he was dead for three days. It's not really relevant how the ancient Jews counted time. This is not a correct statement based on the way we count time in the modern sense.
Modern Day, he was "dead" roughly a day and a half, give or take.
Now, that being said, I should add in that I added quotations over dead for a reason.
Making the assumption they nailed Jesus up on the cross at 9am as you did, and took him down at 3pm is a problem. Roman style crucifixions were known to take
days for the victim to die. It's not out of the realm of possibility that he could still be alive on the Sunday even if he was still up on the cross (no doubt he'd be in very poor health by that point).
However, if you pull him down 6 hours after you put him up, I'd say odds are actually pretty good that he wasn't dead yet. His other injuries sustained was a flogging, and being stabbed with a spear... two injuries that are serious, but certainly survivable, especially for a healthy man in his early 30s.
I have no doubt he would have sustained serious injuries during this story, but we have many examples of people surviving similar injuries, or significantly worse injuries that are said to have been inflicted on Jesus.
So, in short it's plausible they pulled him down when he was either playing dead, or they mistook him for dead... he spent a day in the cave semi-conscious but alive, then came staggering out on the Sunday once he'd had a chance to recover some strength.
Personally, I think the most implausible aspect of this story is that the Romans would have taken him down off the cross on the Friday 6 hours after they put him up. Roman practice was to leave the body up on the cross until it rotted away. The fact they would have taken him down and put him in a tomb is very doubtful.