I put both of your questions on a spare host, because I couldn't answer either from memory.
On colons: xe creates a key literally called auto_poweron:true with an empty value, so the entries you saw were three separate keys and none was the real auto_poweron, and typing false afterwards just made another one.
It exits 0 and prints nothing every time, which I'd call a rough edge rather than a feature, though other-config is free-form by design so I don't know that the CLI is meant to validate keys at all.
On the sleep: with rc.local executable and no sleep, it fired at 16 seconds of uptime and xe appliance-start came back with Error: Connection refused (calling connect ), exit 1, in under a tenth of a second. The VM stayed halted and nothing was reported anywhere, since rc.local has no terminal to print to. rc-local.service only orders after basic.target and network.target, nothing toolstack-related, while xapi-wait-init-complete.service took 30 seconds on that host, which is presumably why 60 was barely enough for you.
There's an xapi-init-complete.target that looks like the right thing to order a unit against instead of guessing at a delay, though I haven't tried it so treat that as a lead rather than advice.
The executable-bit half is in the docs now, it went onto the troubleshooting page after you reported it: https://docs.xcp-ng.org/troubleshooting/common-problems
