So mods will be required to post a comment for removing a post, removing a comment, and banning a user. What about marking as spam?

Marking as spam can still result in the mod replying "Removed because it's spam". This is what I do when I remove spam posts, I warn them in the comments "Ads go in /s/ads please, last warning" and then when they do it again, I ban them and comment "Banned for not following site rules, ignoring spam warnings".

Something like that. I think every mod action that censors someone else should leave a paper trail.

And wanting to ban a user for abuse of mod mail/harassment wouldn't really have a place for the mod to comment.

That's a fair point, but they should just block that user then. That's not justification to ban someone from a sub, imo. They have to do something public to get banned from the public place. Messages between users are just between those users, and shouldn't affect people being banned from subs and so on.

I suppose we could instead show "[deleted/removed]" in the main post view like how it is now, but then if you view it through the modlog you get the original content.

Yeah we can do it that way until we get the spoiler system working. I'd like it to be visible right along with the rest of the comments. I don't want it so a user has to dig through 5 pages in order to see what happened, that'll ensure only super-motivated parties will take enough interest to do anything, which will invite corruption.