BlitzMax + MingW + MiniB3D = Blitz3D ?
Yes and No.
Most functions are the same and are used the same way, but not all. One example is the terrain function, only Klepto's extended version has this and it's not possible to change the height in the terrain directly.
There is also some difference in the programming itself so it may also depend on which language you prefer.
I like the BlitzMax language, and I personally think this is the future way of programming, so I would rather think about buying Blitz3D SDK if I really want to create something what I cannot do in BlitzMax in combination with Klepto's extended MiniB3D.
In the end I think it's possible to get the same results on the screen and create the same game in both languages, and every language there are some sollutions you must find first before getting the same results and the only difference in endresult may be that there may be some difference in speed (framerate) depending on what you want to do.
OpenGL not usable for casual systems?
I think he might refer to Intel graphic chipsets, or other unknown brands. There are some graphics cards which may show some problems in combination with OpenGL.
Best thing under Windows is to let the user him/herself who plays your game choose if he/she wants to use OpenGL/DirectX.