I'm teary-eyed reading this. I can see why God chose each of you very special ladies for this purpose - you are all amazing! He could not have chosen better. These ladies really have some incredibly wise advise that you can read and reread and ponder, Starla. I would pray, pray, pray about it - every single day. You have some time before next year, and I do believe the Lord will make clear His will.
I think it would be helpful to figure out how much "help" you can count on. Your mother-in-law sounds open to helping with your ds, so maybe take a look at PHFHG, and see what she could do that would help the most. You can then show her and get her take on it. I think she sounds like she'd be open to continuing helping, and would really enjoy it, but if you knew for sure what she could do, that may help you visualize your days.
Will your dd still receive OT and speech services? If so, would that be at home or at the school? My ds had speech therapy for several years, and the speech teacher came to our home. This was a state funded program, so I know it probably isn't available everywhere.
MeadowWay made a wonderful point about schooling 4 days a week and having the fifth day to do something on her own with her dd to de-stress. I think this would be a good thing for you to plan for yourself too. As homeschool moms, we always say we can't do that, don't have the time, don't have the resources, but usually that is not true. With a little creative planning, it can be done, and finding that time to de-stress is often what keeps us homeschooling in the long run. So, how could you carve out some planned time each week to simply de-stress? An afternoon? An evening? A weekend day or night? Could dh help somehow, or is he not able? My dh travels all of the time, so he does not help this way, but we did decide together to hire a homeschool gal to watch the dc an afternoon or two each week. This is when I run all my errands and do any appointments I have. We also try to go on a date together at least once a month. I also try to do something with my sister once a week, often for a few hours in an evening. These few things help me de-stress and make a world of difference in my weeks homeschooling. I work part-time, and my dh travels overnight many evenings, so these "helps" are incredibly important to me. I think you will need to plan time to de-stress - how could you best do this?
LHFHG will be fine. It is active, has short lessons, is engaging, and is an excellent program. I'm not concerned about that being a good fit - I'm more concerned about you. So, yes - I do think you can do this!

I do think God will equip you to do it!

I do think LHFHG will be a great guide for your dd!

I do think you've received wise counsel here in the ladies' advice!

BUT, I do think you need to plan for "help" to be in place, and a little research on what everyone is willing to do to make this happen would go a long way to making the year go well. I am praying for you in this decision, Starla. May God continue to be your guide, and may He make clear His will for your decision!
In Christ,
Julie