Five minutes was about my son's limit for handwriting also and it still is even though he takes piano lessons and he did use a squeeze toy to strengthen his hands.
My son would occasionally reverse letters and numbers even after the age of 8. Sometimes he would do math and tell me the right answer as he was writing it but his 5 would be written backward and looked more like a 2. He didn't do it all the time, just occasionally. He never did it when the OT was testing him and he didn't have trouble learning to read, in fact he taught himself to read at 2 but couldn't read for very long because his eyes got tired quickly but would continue to identify the words if I spelled them out for him.
In piano his teacher has always had to highlight the right and left hand notes that are played together because of some strange vision issue that I have never been able to figure out and he would sometimes play a chord with the top and bottom notes reversed--some kind of vertical reversal thing and it tends to happen when he is getting a migraine but there are other days when he plays really well even though he didn't practice at all for a week. When he first started piano the teacher had him using an additional book that required writing but it was so hard for him to write small enough and in the lines that she stopped having him use it. She said she could teach him without using that book.
He couldn't practice as much as other kids could because of the pain and fatigue problem so his handwriting never really became as automatic as it should have but it is legible enough that if he has to fill out a form at a doctor's office he can do it. He will just have to type things that require a lot of writing.
