Good read. I recommend taking the tjme to read it all. But, essentially, it recommends adding it at the beginning, when fermentation is going strong, and again when the SG drops further, but theres no perfect method that I have been able to find.
I used this protocol. I put my first mead together back in mid December (I've got 3, 1 gallon batches going), and they're still chugging along. I would pop the airlock, sanitize a spoon, and then stir the additions in, or stir to degas.
As long as fermentation is still going, removing the airlock is pretty low risk. The co2 produced by fermentation pushes the oxygen out. Just make sure whatever you use to measure the nutrient is sanitized and the time of the airlock off minimized and the risk should be almost negligible.