BBM:But you can backdate an existing post. (You have to post, then backdate.)
http://www.tumblr.com/docs/en/advanced_post_options#backdate-can
If someone had HE's phone, they could have done this with the last posts since no one was looking for her until the 19th, unless someone actually SAW those posts at the time they were posted. Or if they had her password, they could also do this.
In addition, as I mentioned previously, you can change your time zone in Tumblr, any posts you make will be at that time zone. Then you can change back to your current time zone and the new posts will be at the current time zone. The older posts don't change time with the time zone change. Someone would have to be thinking about this, but it can be done with minimal effort.
As to whether HE might have had any posts in her queue, that's anyone's guess. I know several people that randomly queue things depending on their mood. (maybe finds several things they like, but don't want to repost them all one one day, or has several thoughts at once and want to space them out)
But, as I said before, I don't trust Tumblr.
ETA: Nice SM activity list, BTW