I'd try a Spokey spoke wrench. It's advertised as grabbing all four corners of the nipple so they are less likely to round out but, honestly, I've never used one.

Spoke prep is one of those topics in which you are likely to get several strongly held but mutually exclusive opinions. I've used linseed oil in the past but it's messy and seems to work best if you give the wheelset a week or so to cure. I've used bee's wax for my last several wheelbuilds and I'm planning to use bee's wax on a fixed gear wheelset that I'll be building in a week or so. I like the way the nipples feel when I'm building the wheels and I liked how they felt when I disassembled a wheelset. It's labor intensive. I warm the spoke threads with a hair dryer and roll them on the bee's wax block.

FWIW, I went through the exact same thought process for my fixed gear wheelset project and decided to do the opposite. I spray painted the CXP21 rims yellow but I'm using brass nipples. I'm excited to see how it goes.