I was a full member before but since I deleted some of my unnecessary posts, I became a member with activity of 112

I'm unable to even delete my current avatar due to that reason.

Now my question is, since I have a post count of 189 (after deleting the unnecessary posts), how did I become a member from full member?

WTF! How can I gain it back? Will I have to post consistently for 14 days now?

Yes, you are too hasty.
If your activity has not been updated, you just have to wait an hour for it to do so. 

Interesting, I wasn't aware of that. I have been busy lately myself trying to report those pesky bots. How many number of posts to accuracy do you need to become promoted as Staff?

There is no magical number as shorena will probably agree (though he's probably long overdue in getting modded in my opinion)