What I did was take the backplate off the chromebook and took the battery out for about 15 mins. For some reason that worked for me and I was able to access dev mode. It probably depends on what model you have.
I'd advise being careful. We get to keep ours since we paid $300 for them so I wasn't as worried messing with mine.
We don't get any of that lol. $50 for every repair.... even a $15 backplate...