I've heard of it being done but I've also heard of it being refused. It's all going to be down to the immigration office in question (and perhaps even the individual immigration officer within that office)
Highly unlikely you will be able to continue to do this over time. Once will be fine, you might even get away with twice, but I don't see you getting a third time
That wasn't an extension, it was an under consideration period pending approval of your 12 month extension of stay (basically its the first month of your extension). Did immigration not put a slip in the back of your passport with a date on it? If not have you previously done a 90 day report?