Caveat I was trained in physics, not social work, so I can give you some general thoughts from my perspective, but someone in your field will give you better advice. I also hope you get multiple answers from this so you get a variety of perspectives to think about, since I am sure there is no single right answer to this kind of question.
In fact, before getting into my thoughts, my number one piece of advice to you would be to seek out someone who has done a similar path, explain the situation, and ask for their advice. Ask your professional reference what they think, since they have direct experience with this program. Reach out to the person who they wrote a letter for and get their take on how important the various factors you are weighing are.
OK... with that out of the way...
I think your professional letter writer, and a letter from your English professor who supervised you doing very successful research, are no-brainers and will be great letter writers. My normal advice for picking the third letter writer would be to say that you should prefer letter writers who can speak in detail about your work, over letter writers who are superficially "more relevant" but who can only write a generic letter.
However, I keep bumping against this part of your question:
they will be considering the mark I received in my research methodology course for the social science aspect, for which I received lower than their recommended cut-off and it said our probability of getting admitted would be lowered.
This seems like a serious point you need to address. I think you can partly cover this by doing the following things:
- Explain in your personal statement any mitigating factors that led to this low grade, and discuss ways in which you have demonstrated the skills covered in that course that are not reflected in your grade. For example, talk about your successful research in English. While the volunteer work isn't research, you can probably use it to explain you have relevant experience and a demonstrated interest in social work.
- Being up front with your letter writers about this gap in your CV and asking them to talk about your potential in this area.
Now, my advice from this point depends on exactly what it is that was covered in this course, and what is meant by a "related discipline". If "English" is a related discipline and the research skills have a large overlap with English research (eg, the research skills are mostly about reading and writing papers), then I would say that using English professor #2 as a letter writer is a good choice since, based on your description, they are the professor you feel you know 2nd best and will be able to talk about relevant experience.
However, if there are key skills you were supposed to demonstrate that aren't covered in an english degree (again IANAS, but for example this might involve some kind of quantitative data analysis or experimental design), and/or if "English" is too far from social work to be a "related discipline," then I would absolutely not use two English professors because it will just look like there is a glaring gap in your experience that no one can explain. In this case I would definitely reach out to one of your sociology professors and see if they can write you a letter. The interdisciplinary course seems like it could be a good option, given what you wrote.
One other thing to consider here, that your professional letter writer can potentially help to fill in some of the gaps. Again this depends on how important this research skills course is and exactly what was taught in it. Presumably the volunteer work cannot help you very much with demonstrating research skills. But if you can use your English writers to cover relevant research skills, your professional writer can potentially help to fill some of the gaps related to sociology and social work, depending on the nature of the work you did.
Additionally, I have a few comments on some points you raised in your question that are independent of the point of view I expressed above...
it was about serial homicide, so I don't know if that looks too pretty on an application given the subject nature for the field of social work, even though arguably it did examine sociological/psychological context etc
I would not worry about the content of the sociology course. The professors in the program you are applying for are grown-ups and can handle a professional discussion of a relevant academic discipline, even an unpleasant one. Don't worry that you don't have courses in the specific subdomain you will focus on in your masters, just honestly present what relevant work you have done.
also think this professor might remember me because I spoke to him during an office hour and he actually added me on LinkedIn a while after I finished that class, and I was surprised because he added me meaning he probably remembered who I was.
I wouldn't assume they remember you or spend time trying to read the social media tea leaves, just reach out and ask them! Send them an email and ask if they'd be willing to chat. You don't even have to advertise the meeting as a request for a reference, you could try to set up a 15-20 minute meeting to get advice on applications. Lay out this situation to them and see if they have any advice. You don't need to commit to asking them for a letter to check if they remember you and if they think they could write you a good letter, and even to see what they think about having two English professors write the letter.
This professor seems pretty well versed in his field too - he had major projects and even has a TED talk video up.
That's nice, but their ability to speak about your work matters much more than their qualifications; they are not the ones applying to the program.
I am still concerned for the above reasons, but also because I am currently in the process of scheduling a meeting wwith the aforementioned second english professor and it might look bad for me to back out and tell him I don't need him anymore to switch to someone else.
Don't worry about this. You need to do what is best for you. Your professors are grown-ups and understand things change. Changing references is a common thing.
I didn't feel like I could use other Sociology professors because they probably don't know me from the big class sizes in the upper years,
This is a good reason not to use those professors.
and my marks for the upper year Soc courses were all mostly lower than my marks in my upper year English classes.
This may or may not be a good reason not to use these professors. You might not be able to directly compare Soc with English grades; perhaps Sociology grades just tend to be lower overall. The standard should beif your professors know enough about you to able to discuss your work and say positive things.
in fact one of the profs I had for a decent course was pretty old so Idek if he works there anymore.
Don't assume this. If you feel they can say relevant and positive things about you, check if they are still around!!
I'm betting on the fact that English literature majors are known for excelling in graduate programs like Law and Teaching (statistically Eng majors do the most well in LSAT scores next to Philosophy majors) so I'm hoping that admissions recognized that my English skills show that I'm a good communicator and critical thinker which would help be a good social worker.
This argument seems pretty weak to me; you're attempting to generalize the performance of a broad class of people ("English majors") in different fields ("Law and Teaching") to your case and your field. While I think it's good you make the argument in your application that your English degree gives you good communication and critical thinking skills that can help you be a good social worker, I think (a) you should aim to be as specific as possible as to what communication and critical thinking skills you have and how they will help you in social work, and (b) you shouldn't bet on this argument to cover gaps in your application. You should be active in addressing those gaps by writing a statement that makes the best case for yourself, and talk to your letter writers about weaknesses in your application and implement the suggestions they make to you.