Hi,
In the case of installing MySql as a service, you should be able to see a log of errors in event viewer, if not, you can try running the server directly from the command line, for example openning a console at dir <mysql-basedir>\bin and running the following command:
mysqld --no-defaults --standalone --console --enable-named-pipe
(alternatively you can you past the command in from the service properties window, tab general, right below of "Path to executable"
That should tell you more details of why I couldn't start.
Then you can paste that error message here (could be permissions or a missing file).
On the multiple my.ini files, they are read in a certain order and as soon as one is found, the rest are ignored, the order is documented here:
http://dev.mysql.com/doc/refman/5.6/en/option-files.html
If you already changed the datadir and can start the server, you can check
show variables like '%datadir%'
and get actual folder being used for datadir (could be blank if is using default).