Does this mean that we are not allowed to post other OC’s content here without their knowledge/consent?
Yup, if we find out about it, we will remove that image. repeat offenses will result in suspensions or bans. Keep in mind we have no way of knowing whether or not you have consent, so we will assume that you do unless we have reason to believe otherwise.
what about crossposting a post that an OC has made in a community to other communities within this instance,
Super easy to get permission for that, super easy for OP to call you on it if you don't. Just ask!
What if the person/people involved in the post are not members here of this lemmy instance, but elsewhere in fediverse or websites elsewhere - does it make it ok to post their OC then without consent simply because they are not “members”?
Nope.
What about unattributed photos/images grabbed off the web. How do we get consent?
So this is a great question. The very base level of consent we're looking for is "is this person OK with being naked on the internet". If we think that that's missing -- if it's revenge porn, leaked, deepfake, straight up noncon, or if it's ambiguous -- then we will pull the image. If we think the person is OK with being naked on the internet, then we'll probably let it pass unless we have more info. Like I have no idea if that person has licensed this photo CC share alike or all rights reserved, if they want people to spread it around or they don't, and I'm not going to bother investigating because I have a life outside of this site. If that person ever pops up and says hey, this is pirated content, please remove it, I will comply.
[Addition] Are moderators primarily responsible for enforcing this rule and banning/removing content posts? Can moderators be reported/removed from comms for NOT enforcing these rules for posters and by allowing posters to ignore this rule and allow such content in their comms?
Yes, and eventually yes though we would try to work with the mod to solve the issue first. Removal would be a last resort.