Has my MacBook been hacked?
Yes, there's always a possibility that a laptop has been hacked. Are the observations you have cause for concern - not really.

macOS contains a feature named "Power Nap" which allows the Mac to wake itself up from sleep to perform routine activities for a short while, and then go back to sleep. This is stuff like receiving new emails, syncing calendars and iCloud documents - or when connected to a charger - downloading software updates, taking backups, etc.

You can read about the feature here:

https://support.apple.com/en-bn/HT204032

Regarding SSH remote host identification changed: You'll need to contact the owner/administrator of that host to confirm whether or not the remote host identification change is a problem or not. It can happen for completely benign reasons such as reinstalling the computer, changing SSH configurations, etc. 

In addition, it doesn't mean that your computer has been hacked. Rather it means that someone could be tampering with the remote computer or the network in between.