git revert --no-commit without staging

There is no single command for it. As you already noted, you can combine git revert -n with git reset to get the reversion undone in the index.

Besides that method, you can use git apply -R to "reverse apply" a patch, and you can turn a commit into a patch with git show, so:

$ git show <rev> | git apply -R

has the same effect, with one important (but subtle) difference. Let me quote from the git revert documentation and add my own emphasis:

-n, --no-commit
Usually the command automatically creates some commits with commit log messages stating which commits were reverted. This flag applies the changes necessary to revert the named commits to your working tree and the index, but does not make the commits. In addition, when this option is used, your index does not have to match the HEAD commit. The revert is done against the beginning state of your index.

In other words, git revert -n operates in, and on, the index, with a side effect of updating your work tree. The git apply command operates (by default anyway) on the work-tree only, and in fact can be used outside a git repository.