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.
No. Auto Call Recording is not available but you can download Auto Call Recording apps from the google play store and can do auto call recording.
Yes, Motorola Moto C does have front camera flash.
Yes, you can. But right now back panels are not available in stock, since the model is just launched.
No. Moto C does not have the gorilla glass protection. But the display is quite strong than many other phones.