Hi Testtaker,
Thanks for the question! There's several different aspects of your question that I want to address, so this may feel a bit scattered. Please bear with me
First, let's talk about the sufficient and necessary elements here. I think that you've gotten a bit confused with general conditional reasoning vs how Assumption and Justify questions work. We know this is an Assumption question. Assumption questions, by definition, ask you for something the author considers necessary for the conclusion to be valid (and this is why the Assumption Negation technique works on these questions). So, in looking at
every Assumption question, you are
always looking for this general necessary idea; it's not something special to this particular question. Thus, when you mention "another way I could've eliminated B," that approach would always work in a question like this—its universal to Assumption questions. Does that make sense? Please let me know, because this is critically important to understanding these questions. Also, if you are in one of our courses or if you have the LSAT Logical Reasoning Bible, let me know, and I'll refer you to some specific areas to look up to gain more info about how all this works.
Next, the sufficiency idea is worth examining as well, because in a Justify question the correct answer is sufficient to prove the conclusion.This is why someone would talk about sufficient and necessary ideas whenever an Assumption or Justify question was under discussion. This doesn't mean that the correct answer actually contains conditional reasoning or conditional indicators, just that it works in a way that serves as sufficient to justify the conclusion. Again, that's not special to this particular question (in fact, the language used by the question stem in any Justify question will convey that the answer choice is sufficient to prove the conclusion). Let me know how you feel about that, and then if you have any of our materials, I can refer you to some useful areas that address these points.
Ok, next you mention that "My inability to distinguish between necc and suff. conclusions is a reoccurring problem that I face..." Can you tell me a bit more about this? Is this something that occurs in just the most challenging questions, or is it the easier ones too? Does it occur when there are no indicators present, or does it occur in just about any situation? There's actually a number of posts I've made on this forum alone that I can refer you to, so let me know and I'll do my best to help out.
Last, let's talk about LR diagramming for a second. Diagramming is a really useful tool, and there are times when you have to know how to diagram quickly and efficiently. But, diagramming is also something that can't be applied to every problem and is often overused, and there are many instances of problems that contain conditional reasoning where I wouldn't diagram them. When I first read through this particular problem, I didn't consider diagramming any part of it. There's a big leap between the premises and conclusion (we go from talking about an emotional connection to increasing the amount of money), and once I saw that leap and saw that it was an Assumption question, I immediately began seeking the answer that matched that jump, which is (E). Answer choice (E) is indeed necessary for the conclusion to be true, but that conditional element is present in abstract structural terms, not concrete conditional diagrams (if anything, I actually feel like there's more causality here).
When I looked at (B), I had some of the same reactions that you had: I'm not as worried about what most charities have done, because what's already occurred isn't essential to the author's argument; it's about what most charities could probably do. This is borne out by the fact that if you negate answer choice (B), it doesn't attack the author's argument. I didn't diagram (B), and I wouldn't diagram it in hindsight.
So then, why is (B) being described in terms of being sufficient? Well, I can't speak for the author of the explanation you read, but I'll take a guess at what he or she was thinking in saying that. Answer choice (B) does strengthen the conclusion and help the argument. That said, I personally wouldn't say that this answer is sufficient to justify the conclusion, and the explanation you read was probably being a bit fast and loose with terms. A Justify question is kind of a "perfect" or "super" Strengthen question, and so, if we were to put it into conditional terms, an answer that strengthens the conclusion is heading towards the sufficiency side, and I'd surmise the writer was just referencing that sphere of answers when he or she said that. But this isn't a sufficient answer under the terms we use for Justify/Assumption questions, and your writer probably didn't mean for that to be interpreted in strict terms.
So, there's a lot of different things floating around here. Please take a look at what I've written above, and if you can provide me with some additional information, I should be able to help you out even further. Thanks!