Hi, just saw your post and didn't want you to go unanswered.
I have no idea wrt your midcycle headache. Someone else may be able to help with that one.
With respect to ovulation questions:
Ovulation normally occurs 12 - 16 days before your period so a regular cycle will make it much more easier to pinpoint when it may be.
The luteal phase, the time between ovulation and the start of your period will rarely fluctuate. When your period arrives late it is most usually because you have ovulated later than usual.
Through looking at a number of clinical studies, NICE stated It has been observed that most pregnancies can be attributed to sexual intercourse during a six- day period ending on the day of ovulation, with the highest estimated conception rates associated with intercourse two days before ovulation.
My understanding is that sperm needs to undergo a process called capacitation in which the head changes shape to make it able to penetrate the egg. This is induced by exposure to the female reproductive tract and takes some hours. I suppose that is why conception rates are not as high on the day of ovulation than compared to before ovulation. The egg does not live very long, only up to about 24 hours. Sperm can live for a good few days, 5-7 I believe.
The production of EWCM (the egg white cervical mucus discharge) is triggered by increasing oestrogen levels. This occurs before ovulation. A peak in oestrogen triggers the surge in leutinising hormone (LH), which is what causes the maturation and release of the egg about 24-48 hours later.
This probably means that you ovulate at the end of the period in which you have EWCM, but you are fertile throughout its production. It's more sperm friendly than normal secretions allowing the sperm to pass through the cervix much more easily.
HTH