Hi Guys, I did not see this tread till now. I can try to give some advise…
10 inch hole is good enough for 99% of the fish you will catch.. It helps to drill at a heavy angle in one direction and then straight down to knock the sharp edge off the bottom of the hole. This will help the fish slide up the hole rather than be straight up and down. Otherwise yes, a three hole set like a clover leaf is good too but not needed most the time unless you have an 8 inch auger.. Better safe than sorry if you are using a 8 inch auger.
You do not have to grab them by the gill. We have caught a lot on jigging spoons with trebles on. So if I have to avoid the mouth I go for two other spots. Right in front of their mouths behind their barbels is a hard bone that cuts back up into the mouth cavity. That works good for a grab point. Another good spot is over the top of the head where the gills meet the back of their head. Your will reach across each side with most fish under 55-60. Take your time when they get to the hole.. Don’t force things anyways.. There mouth is like the rubber on a tire. You don’t loose many if you take your time.
They are very hardy. Like said you have two issues… Freezing and holding them the wrong way. Keep them in the shack while you unhook and measure the fish. Pictures do look better outside but not worth harming the fish if it is really cold out.