I'll have to see if we can rename this forum. No sense having it be version specific.
1. Yep; you can see all the commands with a core.help. I poked into the software manager and there's a 5.0 and 5.1 API guide doc posted under both ePO versions, and the sizes are different but a cursory peek doesn't show a 'what's new' section or anything like that.
2. Generally yes; there may have been a few changes or tweaks but should be additive.
3. There's a Web API guide posted in the software manager underneath Management Solutions => ePO 4.6/5.0/5.1. It should be posted on the download site as well but it's easiest to grab from the sw manager imho.
Thanks for the answers, Jon.
And for looking into the community renaming.