Learn English – Volunteer or Voluntary Organisation

I couldn't find an authoritative source, but I found several items that touched on the question. The basic differential is that "who" refers strictly to people, and "that" can refer only to anything else:

She is the woman who raised me

not

She is the woman that raised me

Conversely,

There is the school that educated me

and not

There is the school who educated me.

The conflict arises here because you're using these companies anthropomorphically - you're happy with them, you're indebted to them. In reality, you're happy with the people AT the companies / charities, not the companies / charities themselves.

So I would go with

I am forever indebted to the charities that helped me.

or

I am forever indebted to the people who helped me

I think it would be slightly odd if there were an 'industry standard' for this one. Even odder if the general speaking / writing public both knew of that standard and actually used it.

As noted in a comment against OP, you can't check this one with Ngrams because that doesn't support the slash character in search terms.

But you can Google for

"b/w" movies which reports 6,650,000 hits, and

"b&w" movies which reports 4,570,000.

If I add the word american I get 2,960,000 as against 2,310,000. That's much the same ratio, which may imply there're no particular tendency for Americans to favour one term over the other.

I can't restrict Google to US sites only, but I can restrict it to UK only. That gives twice as many hits for b/w, which may suggest that we Brits prefer the slash form. I know I do.