[Not sure who's listening, as this is a 4 year old question posted by someone who no longer seems to have an account here, but... ]
... can an Arduino be programmed in C
without using C++ and the other included class libraries?
You absolutely can use either C or C++, and make use of class libraries or write your own functions for either or both of computing tasks and managing your external hardware.
Arduino be used as a general AVR / microcontroller development
platform to learn embedded C programming?
Yes, for the reasons above, and more. Embedded systems differ from other computing systems by having deadlines the must meet. These can be soft deadlines (try to keep visible delays to a minimum so you don't annoy the user/operator) or hard deadlines (if you miss one, something could fail or someone might die). They come in different scales: my home thermostat could be capably run by an Attiny "with one pin tied behind its back". :) Other systems might contain many large-scale chips or a mix of both, networked internally to one another. A mid-scale example of this is an automobile engine controller. It runs your signals, entertainment system, ABS brakes and traction control, and more, all while opening and closing injector valves and firing spark plugs with precise timings, at up to 125 times per second. And that is for a pretty normal, family-car, "street" engine.
The Atmega328p (Uno's MCU) is sufficiently over-engineered to tolerate occasional misuse, being over-clocked or supplied a lower than spec'd voltage, has several ways to update/replace its program, is easy to hand-build into a custom micro-controller board, and after all that abuse, is cheap enough to replace if you do manage to damage one.
Documentation is readily available as are online communities like this one, to help with beginners questions, unusual questions, esoteric uses that maybe only a handful of people have tried or know about.
So, tl;dr: yes, I'd call it an ideal platform for everything you asked about.