First of all even with decent OpenGL/3d programming knowledge it will be pain to make. I can give you few examples of homebrew brush editors but they never got any far and are HARD to use, Quark and Hammer 3D (with its other deprivations) rule the market and hammer is used even by professionals (though bsp format by itself is horribly outdated) I think it would be plain easier to develop plugins for more geometrical shapes in constructor or improve existing ones instead of wasting so much time trying to make tool that already exists, believe me you will run into MANY problems along the way, brush editors are more than rendering triangles, it's math, lots of it that is.
Constructor works the way you just explained your tool to be, you just have to configure views and have some experience or ask someone around here, there are few people here who know how to make messy stuff, thing I love about constructor is that you can edit things even in 3D view and texturing is really easy, it's way more intuitive too. The core problem with Constructors moving platforms is 1. no multibrush MP's 2. slightly buggy and crashy 3. less intuitive.
Now talking about direct exporting, i know about this trick ever since ive started using constructor.
Best part is you can configure exporter whatever you wish, you can use .csx format (constructor will export it to map if asked) which doesn't make triangle geometry disappear on map load. You can say where stuff to export etc etc.
Complicated geometry? There are 2 choices: aether blender it up (trust me, it's not easy there eather) or use constructor and some trickery, with higuys scale two zero plugin (
higuy.me/scaletwozero.php
) it has never been easier, just make complicated things and pray that MBmap2dif_plus won't leave holes in.
I can't ditch quark too, it's program with many years of stability, constructor is something that was slapped together quite quickly and it's a mirracle it even works so well, it is buggy at times but it's the simplest brush editor you can find around. It really depends on your workflow though, some love quark more than constructor because they have different workflow.
I'm not saying you shouldn't make it, more like suggesting for you to not reinvent the wheel and do bit of research, maybe ask someone here who uses constructor or quark quite often about what they seemed both programs lacked when making MB levels, of course coding anything is great for education but if you want to make this into serious tool you may end up putting lot of work for nothing.
IMO i think only thing constructor lacks (excluding better MP hadling) is displacement's, they can be sortof manually done but it's hard to do them so that they feel natural. With proper tool it would be easier to make more natural MB maps.
EDIT: here is also this tool, it costs a lot, it's workflow is very interesting but that has 3 years of stability now.