Let's get the basics clear:
It is not your marriage. It is not your husband or your life. Unfortunately, there is only so much you can do without getting verbally abused yourself and probably getting banished from the house.
Having said this, it does not mean you should keep quiet. You actually SHOULDN'T keep quiet. That is just inhumane on your part. I am not badmouthing your boyfriend for not wanting "conflict", but some sort of interference is needed. His discomfort towards being in the middle of a conflict should not stop you from saying things. What exactly do you say though?
How should I respond to verbal abuse I'm a witness of? (My boyfriend doesn't want conflict so I am also trying to respect that)
You intervene. You don't talk about the abuse itself, you don't talk about the particular situation in hand. Just intervene with something random. You make a joke based on the verbal abuse. A casual off-handed statement. Nothing that hurts Bob. Nothing that makes him feel like you're accusing him of being an A-Hole.
The purpose of an intervention in the form of a joke is so that the tension goes down a little bit and no feelings are hurt! You could also try to casually walk up to Bob and stand next to him and mock Lisa a little bit along with him. And then walk over to Lisa and maybe give her a hug and say, "I'm just kidding Lisa don't kick me out!".
If you don't feel comfortable enough doing that, call Bob over to you and ask him something unrelated. Get him out off that situation that he is putting himself in. The minute he calms down for a second, he will realize he was being unnecessarily mean to Lisa.
Is what is going on none of my business and I should just let it go?
It is definitely none of your business. Should you just let it go? Different people would say different things. I am personally a very empathetic person. I could NEVER let it go.
I was 11 years old. My neighbor is horrible at beating her kids. She had 2 boys. Age 8 and 5. This one night in particular, I could hear the mom beating the living soul out of both her kids. I could hear frying pans flying around in the house literally. I could hear blows landing on the kids. I went to my dad and he said, "none of our business, let her do whatever she wants". So I walked out, picked up a stone, threw it right at their window and walked back in. The sound of the glass breaking stopped the beating for a little bit. My mom looks at me, doesn't say a word to me, and walk out of the house to see what is going on. Obviously, the abusive mom is out investigating the broken glass. My mom acted like she just heard a noise and didn't know what it was; so she came out. Using that as the opportunity, she started talking to her about beating little kids. The beating stopped.
Was my behavior necessarily acceptable? Probably not. I was 11. what do I care about your broken window. I cannot take it when you beat a little kid this hard. There was actually a campaign going on in India for a long time. Abusive husbands are usually not stopped. The government had a PSA going on that said "Bell Bajao" (in Hindi) which translates to "Ring the Bell". The video is of a few little kids ringing the door bell of an abusive husband beating his wife. Nothing else, just ringing the door bell and the husbands stops to answer the door.
That tiny intervention can actually stop such an act.
How can I start a conversation about the fact that they both need extensive professional help which means that they would have to leave the farm alone in order to get the help they need? (Looking for suggestions or ideas about relevant conversations)
How close are you with Bob? How close are you with Lisa? Answer those two questions to yourself first. If you are at least a little close to either, you can talk to them about this. If not, you cannot talk to them about counselling or getting help.
But that doesn't stop you from getting close to them first! Have you ever been able to spend time with either of them ALONE? From what I gather, the main problem here is Bob. You say Lisa has a drinking problem, but that could probably be rooting from Bob being, well, Bob.
So your goal is to get closer to Bob. And by closer, I mean close enough for him to want to vent to you. Close enough for him to not think that you are just a random guest and he shouldn't be talking to you about his problems. Next chance you get, try and talk to Bob about just about anything BUT the issue in hand. Talk about how well farming is working out for him. Talk about what he is growing right now. Talk about all the topics that might not be going so well for him. When you hit the one topic that triggers Bob's venting, that is the topic you are trying to get to. Once you are there, talk more. Ask more questions. Find out why he is behaving like this without asking him.
You are trying to make him comfortable enough by asking him questions that would involve him bursting out in tears. He would eventually start rambling about something and you could then maybe make your suggestions. Make him realize that he is not completely off the deep end and that he can still be happy with his life. It takes a lot of work to get there, but it will definitely work in your benefit (and Lisa's, of course).
If Lisa has a drinking problem despite Bob's behavior, then Bob might be acting like this because of Lisa. So you do the same with Lisa. Whichever way it has to go, it has to go through those steps.
How should I respond to Lisa every time she confesses things to me about the situation without making Bob upset?
You had mentioned that Lisa just received the appropriate paperwork and legal documents to be able to travel. Have you seen them talk about going to Tibet to see Lisa's family since that happened? If not, that is a BIG problem. Lisa might be severely homesick. If it is, she needs to go home and spend some time there. Whether Bob comes with her or not. Honestly, Lisa going by herself would be the ideal situation.
Lisa is confessing about her childhood life and her dad because she is homesick. You suggesting they should both take a vacation to Tibet and blow some steam off would be the most appropriate response to that. Watch your tone when you make the suggestion though:
You need to sound excited! Something in the lines of "Lisa you should totally take a vacation! You should go to Tibet, do all the things you used to do when you were a little kid." - Pick some scenarios out and paint a picture for her, make her excited. "You just got your paperwork too! Aren't you excited that you can now go visit your home and your family again? It should be so much fun for you guys!".
If Bob says he cannot leave the farm unattended: "Well, Lisa why don't you go by yourself. You do know how to take a flight alone right?" laugh about the casual mockery you just made. "I really want this food item you've been telling me about that they sell in Tibet! Go to Tibet and get me some".
This entire thing needs to be as casual as possible. Outside of this being a possible solution to their entire problem, this is pretty much the only thing you can say when she says she misses her home.
All of these suggestions are just my opinions. If any of these won't work, feel free to let me know in the comments and I will change my opinion about dealing with a situation like this.
I hope you help the couple bring love back to their relationship. Its always a happy day when that happens! :)
If none of this is possible because of your situation with your boyfriend, I'd at the least tell my boyfriend I don't want to go to their house anymore. You cannot stop the abuse but you can definitely not have to witness it.