There are many features which make Moto C and Moto C Plus - completely different phones.
01) Moto C has 1GB RAM while Moto C Plus has 2GB RAM.
02) Moto C has 2350 mAh battery while Moto C Plus has 4000 mAh battery.
03) Moto C has 5MP rear camera while Moto C Plus has 8MP rear camera.
But all these also makes Moto C Plus costlier than Moto C. If you are looking for normal usage - Moto C is a better option, considering the price. Both Moto C & Moto C Plus support OTG and also has front camera flash.
Yes, Moto C is a VoLTE mobile phone.
No. Moto C does not have the gorilla glass protection. But the display is quite strong than many other phones.
Moto C has a decent pair of the camera on both the sides. The rear camera has 5.0 MP resolution camera with Autofocus and LED Flash. There are many other options available like beautifying software, HDR, Burst mode,etc. The front camera has 2.0MP and HDR support. The front camera also has LED flash and you can also use display screen as flash.
Yes, you can. But right now back panels are not available in stock, since the model is just launched.