About the second problem, I think it is one of control. Source code access at least gives you a fighting chance.
That said, I have used Qt a couple times, and never dared to delve in its source code, it's just too big. If I have a problem, I just work around it. If you want control over your engine/framework, you pretty much have to have written it in the first place.
Here I agree very much with you. With stuff so complex I can't imagine going through all that code. That's why I don't get people saying "if only I had the this or that source code".
Well, there is a middle ground: when you haven't written that big framework/engine, but could have, delving into the source code in an as-needed basis may sound like a decent option.
Of course, you're hoping your framework is fit enough for your purpose, so that you rarely have to inspect it —if at all.
About the second problem, I think it is one of control. Source code access at least gives you a fighting chance.
That said, I have used Qt a couple times, and never dared to delve in its source code, it's just too big. If I have a problem, I just work around it. If you want control over your engine/framework, you pretty much have to have written it in the first place.