Well! Gosh. My son also has Aspergers and LD. I cannot imagine cancer on top of that :(
Thank goodness your boys have such a wise, centered, and well-resourced family!
A couple of thoughts:
1. Yes, please feel no need to work right now! You will have enough to do for the next while.
2. ...unless doing so helps you feel well and balanced. So, while I wouldn't do a part-time gig for money in your situation, if a part-time gig helps you replenish, have a mental break, etc, feel free! Because YOU will need care and support and respite too. For some people, work serves that purpose. Maybe something with reduced risk of picking up bugs, etc, though, yeah.
I'm sorry I can't input on the questions you posted, I'm in a different country so unfamiliar with IRAs, etc.
I wouldn't concern yourself one iota with the lifestart funds. You are obviously doing all the front-loading necessary for your children to have the best possible go at life. As you note, there will be countless opportunities for them. Most kids don't have a lifestart fund, and your kids will do just fine without this.
Note that MMM essentially recommends *not* handing money to kids. I differ on this count when it comes to housing in my current location and market, as well as re: disabilities, but I think his overall idea is sound. Young, healthy, able adults with a solid foundation are generally better served by figuring out how to navigate the world.
If you feel they do need something, per the one son's disability for example, a term life insurance policy may be the way to go for now. That way you know they have something if you can't be around to pitch in as needed, but it costs extremely little to have in place.
You are gifting your kids perfectly right now: Supporting each to live and thrive and recover from this huge life event. That's all that matters.