Lice - Without question, this is something that BOTH parents should be notifying the other of because it is so easy to spread and reinfect. Consider the possibility though that the reason she might not have told you was because she was embarrassed; many people still associate lice with "dirt/dirty" but lice themselves are not so biased.
Dentist - Again...something that BOTH parents should be notifying the other of...kind of silly to have the expense of two x-rays and two cleanings and potentially dangerous to have two different dentists working on the same problem.
Activities - I think you are being a little anal and setting YOURSELF up for problems with the Judge. Are you really going to prohibit the child from participating in basketball because it ENDS March 23rd or because football starts BEFORE September 21st?!? It's called "common sense" and "pick your battles wisely".
Maybe a nice, gentle reminder on the first two issues is in order...don't make it "THE COURT ORDER SAYS" letter...more along the lines of"
Dear Ex:
I just found out that our son had lice during the summer and while I understand how embarrassing this can be, it is not indicitive of your abilities as a housekeeper. We were BOTH lucky this time, not only could have spread to my home, but worse, it would have come BACK to your home and is an example of why we need to keep each other informed.
We also need to keep each other informed about all doctor and dentist appointments. This is not a "control" issue, this is about our child's health and making sure he is not being over-treated or over-medicated for the same issue. Can you imagine how dangerous it would be if we both took him in to get the same booster shot?
I am looking forward to receiving a copy of son's basketball schedule, I know that he is very excited about the upcoming season.
Sincerely,