The timeline I personally find the most accurate is on p.9 of this thread. See the message from Bwojtach dated 5/16/17. According to this timeline, Tuesday 3/28 was the last time Anna spoke on the phone with her family in Poland. From that point on, there were only SMS messages sent from her phone. So you're right, if something happened, it happened most likely on 3/28 or 3/29. Now, to be fair, it may not necessarily be the worst case that everyone seems to imagine. Here's one entirely plausible scenario: say, Anna had a bad bout of depression and couldn't talk to anybody. So her hubby, not wanting to alarm relatives, picked up her phone and texted on her behalf.
There's one piece of information I recall seeing somewhere here that nobody picked up on, but I find very peculiar. If I'm correct, Anna's husband spoke to her mom on April 8 and told her to call back in two days. Remember, "in two days" means Monday, April 10, when Anna was first reported missing! The questions that immediately pop up in my head are:
1. Is it a coincidence?
2. Why "two days", why not "tomorrow", or "a couple of days", or "three days", etc? Why not "she'll call you back"?
3. Was Anna's mom satisfied with this answer? Didn't she ask "why"? What did he respond?
4. Did Anna's mom call back in two days?
5. Did she talk to the husband? If so, what did he say?
Etc, etc