I can't speak to Neijing, but is it that you're interested in internal training practices? My thought is that any translation from original texts will more than likely be innacurate, vague and/or misrepresented. However, that doesn't mean you'd have any better success reading the texts in their original form, either.
First off, how helpful are other breathing practices to Aikido? I've read alot (and I mean ALOT) of guides on ki breathing. The most interesting one so far is the Nishino Breathing Method. But is there a default means to ki breathing that I can learn and practice outside of the local aikido dojo?
When you're talking about ki breathing, are you talking about Tohei sensei's, ki breathing? Not that it is particularly unique. I've also heard it referred to as budha breathing or natural breathing. There's also reverse breathing methods. From what I understand, different breathing methods are better than others in targeting
certain areas of conditioning. But what they all have in common is using controlled pressure, accompanied with proper intent and appropriate relaxation, to condition and retrain how the body is used.
How necessary is it to be taught inside a real Aikido dojo?
You need some kind of exposure to someone with the body skills that you seek to give yourself. This person should also be hopefully willing and able to teach. Going out on your own without any kind of guidance will likely just have you spinning your wheels. So ... I'd say it's necessary ... to find
someone.
I'm asking because the nearest aikido center is over 25 miles away and I'm not sure I can make such a commitment with my schedule.
This statement runs counter to the very first sentence of your post.

Thanks,
Adam