My goodness, you have a lot on your plate right now!
Hopefully, this will be only a season in life for you, however it is wise to consider that you will likely have quite a recovery period even after your surgery. So, with this in mind, a plan is a good idea.
First of all, it sounds (and looks) like your son is doing a good job with Preparing Hearts.
You can be so proud of what he is able to do on his own and on how he is growing in many areas.
So, take a moment to feel very good about that!
Next, as far as correcting your oldest son's written narrations goes, I would suggest using proofreader's marks in pencil to show your son what needs to be corrected in his writing. This is not hard to do, and I do it with my own kiddos. First, have him read the narration out loud to you. This will help him catch many of his own mistakes. Let him use his pencil to immediately erase and correct any mistakes he catches while reading aloud his narration to you. Next, go through the narration yourself with a pencil. Underline three times any letter that needs to be capitalized. If a punctuation mark is missing, make an empty circle at the spot where it should be. If a punctuation mark is added where it does not belong, circle it in pencil. If a word is not spelled correctly, lightly circle it and write the correct spelling for the word in the margin. If a word is missing, make a small carrot where the word belongs and write the missing word above the carrot.
Then, have your son go back through and capitalize underlined letters, add missing punctuation to the empty circles, erase circled extra punctuation, and erase and correct the spelling of circled words copying the correct spelling from the word you wrote in the margin. After he's done, he can erase your marks, leaving a fairly clean copy without recopying!
This process will go more quickly after you've done it a few times. Plus, it doesn't take your voice to do.
As far as narrating goes, one thing to remember is that the goal of narrating is for the child to retell the passage using words and phrasing he/she has borrowed from the author's telling and to share in detail those parts that stuck in his/her mind. The child may key in on a part of interest and devote more time to the telling of that particular part. A narration is different from a summary in that the child makes his/her own connections and brings those out in the narration rather than trying to summarize the passage by simply hitting all of the key points.
As you can see, with the focus of narrating being on the child's connections, your role is not to have all of the answers, or to correct the child, or to make sure he has included all of the key points; but rather your role is to avidly listen with book in hand to make sure your child is narrating the correct passage and is not wildly off topic. After the narration, you may gently ask your child, "What about....?" or "Could you tell me a bit about....?" however, you don't have to ensure that your child has covered it all.
This makes narration sessions more of a exciting opportunity for your child to share about what he read, rather than a quiz show where he must get the right answers.
This is why the key ideas are helpful, and skimming the text as your child is narrating is helpful too, but having the parent read the text ahead of time is optional.
As far as DITHR goes, there is an option on each of the teaching days to turn to the back of the Student Book and do a written assignment in place of the discussion. This is for days when you are not able to do the lesson as written and need your student to work independently instead. Days that have these options are noted with a small clock above the plans in the Teacher's Guide. In your situation, I would have your child do DITHR only on the days noted in the Preparing plans (which is 3 times each week). Whenever your son has a lesson he is supposed to do with you, instead have him turn to the back of his 2/3 Student Book and do the written assignment noted for that day instead. This will allow you to keep going with DITHR, without having to talk.
For any other boxes that you need to check your son's progress because they are supposed to be "S" or "T" boxes, have him meet with you and have him read aloud the box directions and then answer any questions in the box aloud, or point to any areas mentioned on the globe, or show you any notebook page he has completed, or read aloud any narration he has written. This can be accomplished in a 30 min. meeting chunk in the morning, with another correcting time just for you in the afternoon.
The meeting time will make sure that you have your finger on the pulse of what he is doing each day, will keep him interacting with you without added stress on you or your voice, and can even be done by climbing into bed with you or sitting on the floor next to you on the couch. I know this is true, because I schooled through 11 weeks of complete bedrest flat on my back with each of my last 3 children.
I was only allowed to sit upright 15 min. out of every 4-6 hours and Preparing was one of the guides I was doing at the time.
This short blip of mine health-wise is certainly nothing to what you are dealing with health-wise, but I share in the hopes that you can see that schooling bedside or couchside with little talking is possible.
As far as your little ones go, I wouldn't worry about LHTH right now. Young children can often catch up very quickly and make great gains.
For Beyond, I would try to work to do the left side of the page one day and the right side the next day. This will put school at about 45-60 min. a day for that child and will keep that little one moving forward steadily each day. If there is any way that your husband could read aloud the history pages assigned in Beyond one night and the storytime chapter assigned in Beyond the next night that would take care of the main reading aloud for the most part (if you are doing the guide at half-speed). Then, you could have your older son do the poetry when it comes up with the younger child, and you could do what activities you can from the rest of the plans.
Going half-speed doing the left side of the page one day and the right side the next would really help keep Beyond from being overwhelming. Your little one who is LHTH age could also join in for anything with the Beyond child.
Otherwise, if it feels better to you to just do the 3 R's for the younger pair, you can continue as you have been doing. I wouldn't personally start searching for another curriculum for the littles to do instead though, as it's almost certain you'll end up with it being as much work as it would be to just do Beyond at half-speed.