I'm sorry to hear you are experiencing trouble with Ninjaboard.
You did a good job by installing com_ninja separately. That's the recommended way in case the installation fails at that point.
When you get a white/blank screen this means that there are PHP errors involved. But according to your debug info there seems not to be any error or whatsoever. So, I don't have a solid clue yet.
The method you used for the php.ini and .htaccess, does not solve anything unless your server has Suhosin installed. You can check this by browsing from the menubar in the backend to Help > System Info > PHP Info (tab) and do a page-search (ctrl-F) for Suhosin.
In case Suhosin is installed, you have to put one php.ini in the root of your Joomla folder *and* one identical php.ini in the administrator folder. Make sure you have done that because I couldn't notice it clearly in your post.
If this doesn't help then you can try the .htaccess method.
And if that doesn't help then your host doesn't allow you to use such method because of safety concerns.
In this case you have to contact your host and ask them if the will do it for you.
More info about the Suhosin workaround method can be found here.
In case Suhosin is not installed, there's something else causing the blank screen.
Probably when an installation isn't fully completed, like you mentioned.
Installation failures can be caused by server time-outs or other server limits.
Please check at the same PHP Info page what the value is for upload_max_filesize.
If this is less or equal to 2M, then your upload is limited to a maximum of 2Mb.
This could explain an incomplete installation because Ninjaboard is just a little above 2Mb.
If this is the case then you can try the Install By URL method in de Install Manager:
I hope this helps.
Let us know how it goes.