Unfortunately there’s no way to run two copies of the same application on the same Android phone so in order to play with two different accounts you’ll have to manually go to the app settings, clear the settings for the Blood Brothers app and log with your second account everytime you want to play with it and then switch back.
The alternative to this is to run a second copy of the game from your pc with a software called Bluestacks that emulates the Android environment and lets you install and run applications and games quite easily.
Unfortunately at the moment there’s no way to change Warlords in Blood Brothers.
This is a feature that should really be implemented because when you’re starting to play you actually have no idea what each of the Warlord’s skills do and you might end up deciding on a terrible underpowered Warlord (something that happened to me as I chose the Purple Knife based mostly on her nice looks).
No, it’s not. This is actually normal behaviour for the Gameboy and Gameboy Pocket black & white series.
When you boot the device without a cartridge it will not show the Nintendo logo but instead only a scrolling black rectangle with the Registered mark next to it. Your Gameboy should be in full working order but test it with a game cartridge just to be sure.