For now - new developers and publishers will have to be requested through the forum.
I suspect you may need to keep it that way forever. Even if you write a style guide for it there are too many people that don't read that as is evidenced by all of the game duplicates and titling errors. Whether you do open it up to users or not, you'll need to decide how detailed company names should be and whether to use publishing brands or just the parent companies. There are quite a few companies who have changed their names too, sometimes it is minor and sometimes it isn't. Do you add both or just go with one? Then there are numerous in-house development studios that can sometimes be difficult to track. Here are some examples:
-Team17 Software Limited became Team17 Digital Limited in 2012
-Namco Hometek, Inc. became Namco Bandai Games America, Inc. in 2006, switched their name around to Bandai Namco Games America, Inc. last April, and this April they are going to change to Bandai Namco Entertainment America, Inc.
-past Sega can get a bit complicated as they went through all of the in-house developer name changes with the AM teams and then going back to some of them and then dropping them completely
-Nintendo has had a bunch of studios too, and then there is Nintendo EAD which is all I ever use in my work but it can be broken down farther
Some of us can probably keep up but I think the average user doesn't pay very close attention to company details. I assume most of the duplicates come from some people just adding a broad name to those going detailed (Sega vs. Sega of America vs. Sega of America, Inc., etc.). I usually aim for the most accurate so I'd add the corporate bits like Ltd., LLC, Inc., GmbH, etc., but I'm guessing the majority of people might not. I'm sure you know all of this though as you had to take it offline for such a long time
I imagine sorting through all of the past submissions is a major pain, good luck with that
