I am testing the phone now, so will try to answer some of the questions.
OS is actually pretty decent. comes with 4.4.2. the ZenUI interface looks rather nice. Modern flat design. I like the colour bars. Also you can load other launcher too if you prefer. However, if you want to use it with the tablet, you should stick with the default launcher which is better at switching between the two interfaces.
Battery drain is around 25% for 7 hr usage. Usage pattern is talktime around 10 min, two email account in background with Whatsapp and usual stuff. Slight surfing. No problems getting through a day use. The tablet dock also charges the phone.