Yeah, your itinerary sound good but one day in Mandalay is not really enough for you, better stay two full day and visit to Mingun also. Another, there is no bus from Bagan to Thazi, only need to hire a private transfer from Hotel in Bagan.
Yeah, if you take the route from Kalaw to Thazi, you will get dark on the train, if the train late in case, then, you may miss some scenery. You better take the train from Thazi to Kalaw is great. Just do like that, this is the most suitable for you. Have a nice trip. :)
All the rest is ok but if you have time to take trekking in Kalaw for two day, it's not really worthy for you. better do the two day one night trekking to Inle because the scenery of Kalaw to Inle from the train will not be attractive to you after you took the route from Thazi to Kalaw. So, you should do the hike to Inle from Kalaw if you have time. :)
