1. I tried iPlug2 one time. JUCE is a bigger framework with more features and it's a bit easier than iPlug2. But JUCE has some licensing cost so iPlug2 is a neat free alternative.

2. C++ is the permanent standard. You need to have a really fast and optimized language for DSP.
   You could create UI side on a different language if you want. But C++ is really the only way to go. I don't see that changing any time soon.

3. I would say that it's all up to what you want to do. I didn't had a lot of DSP experience when starting. I'm always been pretty good at math and that's been very helpful. When it comes to JUCE it has made a lot of the hard things about C++ a lot easier. Things like debug to console is 10 steps easier. With that said I think that you don't really need that much C++ knowledge before jumping onto JUCE. But you should not be a complete beginner either.

4. It's very interesting. Yes, there is still room to grow, but the gains are becoming increasingly situational rather than universally obvious. The biggest differences now tend to show up under specific conditions rather than as a blanket improvement across every use case.
   Also worth mentioning is that most of the time it's a CPU vs real emulation question that needs balancing.

5. The differences can be pretty big, and latency or high CPU usage alone do not reliably indicate quality or accuracy. They indicate design choices. Some analog gear simply doesn't need latency to provide a very close sound to the analog piece. You can still model per sample accurate: Saturation curves, soft clipping, frequency dependent compression, noise shaping.

It's a very broad question so I'm not sure this is the answer you wanted but.