1 - Yes, you can run it without a battery
2 - I believe this card does passthrough with monitor switching, it's not a scandoubler. In other words, if you install software such as Picasso96 or CyberGraphX you'll be able to take advantage of this cards capabilities for higher screen modes with supported software (most anything operating system friendly can be mode-promoted). Native Amiga screenmodes will pass through, but not be scan-doubled, so they'll still be 15KHz. For scan doubling native output that you would want to get something like the Indivision ECS and hook it's output into the passthrough input connector, which would allow you to use one monitor.
Obviously disregard all the above if this card actually does scan doubling, not just passthrough.
3 - I own this same accelerator and have never had any problems with games - I use WHDLoad. it can be disabled via the GVPCpuCtrl program.