Welcome,
GLB is free for 2D on Win/Linux/Mac for non commercial use, so you can easily test it (3D/ Net/ Inline commands work for 5min as a demo too). In GLB you can use INLINE to call C++ if you find something that can not be done with native GLB.
1. No native DB commands inside GLB, we got commands for general file access and special for commands for INI files which are really easy to use. IF you really need DBs you need to write it yourself, use C++ via INLINE or use a wrapper like
www.glbasic.com/forum/index.php?topic=1977.02. Yes.
3. No specific terrain commands, but I think all that you could need is still there as commands for general 3D objects. I think I remember that there was code for a generator to generate a terrain from heigthmap.
4. I like the IDE too (most of the times), but like I said GLB is free for 2D, so just test it out.
5. Yeah you can use C++ to create wrappers - so how much C++ do you know? There are a few instructions/examples how to do wrappers in the forum.
Hope that helps you a bit.