To answer your first question, you can't really have the battery status displayed all the time in the Modern UI. There is simply no area dedicated to statuses such as the battery's level or the time.
I use a program from the app store (free) called Battery Level, which gives me a live tile on the start screen with the battery's status, but that's about as close as you can get. The app could use a bit of fine tuning and it requires you to run a couple of .BAT files on the desktop to make it work, but it's useful, nonetheless.
Your second question really depends upon your Internet and external file usage. You're generally okay with only Defender if you don't go to any malicious websites or download things from unknown sources. The same goes for external storage. If you don't use it much and know the source, it's usually okay. If you're constantly swapping files from friends on USB memory sticks or micro SD cards, then you may want something more than just Defender. If you're only going to run Defender, you should be sure to make current backups on a frequent basis, just in case.
Finally, for your third question, the onscreen keyboard only pops up regularly for Modern UI apps. For desktop apps, you must bring it up manually by touching the keyboard icon that should be pinned to your task bar. I'm pretty sure that I've noticed a few desktop apps that will actually launch the keyboard for you, but it is completely app dependent.
As far as Windows 8.x goes, I'm not a big fan. The desktop side is great, but really the same as Windows 7, just without a good way to start applications. The Modern UI side is half-baked and incomplete. For example, you cannot completely set up and administer a Windows 8.x machine in the Modern UI. You are forced to go to the desktop for many, if not most, of the settings.
Since it is necessary to rewrite all apps for the Modern UI anyway, I really don't understand why Microsoft didn't completely abandon the Windows XP-7 code base and start again from scratch. They could have given us a new and modern operating system without the limitations carried over from the past. To maintain compatibility with older Windows applications, they would only have needed to provide a virtual machine in which to run Windows 7 and you'd have both the old and the new. You could be a Modern UI diehard and stay in there all the time, once the software starts to catch up, or stay completely on the desktop side if you wanted to or jump between the two at will.
Microsoft appears to have attempted to give us two operating systems in one, but with the usual compromises, so it's not nearly as good or complete as it should have been - IMHO, YMMV.