The magnets I had at the time were plain without holes and I was using 2 per pole forced to sit in repulsion forming a capital T, I didn't want to depend on glue alone holding them. Pining them around their outer edges to hold them in the T shape would have meant accurately drilling maybe 6 holes or more per pole, with 64 magnets that would be a lot of machining.
The drilling of the magnets was really very easy, but doing 64 of them did take some time. The counter sinking was even easier and literally only took a few seconds per magnet. In your case only half of your magnets have the csk on the wrong side, it would take no more than 10 mins to do them, certainly much quicker than returning them. I tried every type of bit I could lay hands on inc quality masonry bits and they were useless it takes the glass cutting bit, they go through them like a hot knife through butter. It maybe the shape of the bit that makes the difference, they are similar to a spoon type cutter.
This machine has withstood all that our extremely wet and windy weather has thrown at them.
The exposed magnet from drilling was treated with epoxy and shows no sign of corrosion. I didn't pot them, just give them a couple of coats of agricultural paint and they look as good today as when they where fitted.
I have had magnets rust badly in the past when potted, I have even seen the plating on new magnets flake off just from handling them, I paint the magnets on all my machines now and corrosion is no longer an issue.
I used stainless screws as I was worried steel screws would rust and grow in diameter in our wet climate and possibly crack the magnet. I dint think the drilling has hurt them in any way, they have been flying now for about a 1 1/2 years without any problems. I am located in Northern Ireland near the coast and our climate is extremely wet and salty if they were going to corrode they would have done so by now.
At the time I got a good deal on the magnets they where less than half the price of pre-drilled ones so I got twice the amount of magnet for the price.
I will be building an 18 footer this summer and I will do it the same way again it has worked well for me.
Regards Russell