Can someone edit a post (Questions/Answers) which are edited by Community♦ but having spelling and grammar mistakes?
Yes, you can edit the posts without any fear. As you have the edit privilege, there is no need to wait for review too. But just remember that they need to be substantial.
Edits done by Community♦ are of two types.
Just bumping an well received unanswered question to get better attention. You won't find any changes in the revisions.
Edits proposed by anonymous users. Community owns the edits done by anonymous users. The changes show in this case after they are approved in the review queue.
The edits done by community are just the same as the edits done by any user. So, feel free to correct any mistakes in the post.