shiroiuso

My husband's reading level isn't great, but he studied the driver's manual very hard for a week and passed it. I highlighted all of the important bits that would be on the test for him. Maybe a friend or family member can help you that way?
He took the written test instead of the computer test because you can see all of the questions at once, and you have more time to think about the answers.
Alternately, in AZ at least, you can hire a translator to read the questions to you in your language. Florida probably allows that, too.

I submitted barely anything - photos (certainly less than a dozen, and most from the wedding), two affidavits from family and friends, and an itinerary from my husband's visit to meet my family in the US. I just didn't have anything else... no life insurance, no kids, no mingling of finances etc. We were living together but I didn't have any real proof of that because the property belonged to his grandparents. Still, our application was approved with no RFEs.
I say get and do whatever you can that you think will help (and my experience was the affidavits were not worthless), but don't stress out about it too much.
I used that book in conjunction with the forums and forum guides here. The book is useful because it has examples of forms filled out, and the forum is helpful because you can get tips about common mistakes and how to avoid them.

I have experience with this - been in Japan 7 years, earning money but not enough to exceed the foreign income exclusion.
I think you're getting confused because of the particular phrasing, "exempt from filing taxes." As a US citizen, you are NOT exempt from filing a tax return. You are, however, probably exempt from paying taxes on what you earn.
You still have get a record of how much you earned overseas (if you are working for a Japanese company, this will be on your 源泉徴収票/gensen choshuhyo; if you don't get them or lost them, there's an IRS form that will allow you to use an estimate of your income along w/ a letter of explanation as to why you think that estimate is accurate). You convert your Japanese income to USD (Google is fine for this), and then report that number to the IRS. If, like me, you haven't earned enough to exceed the foreign income exclusion - which as mentioned above is something like $90k - you can file a form 2555 to have your foreign income excluded from taxation by the US government. That means your gross income will end up $0.00. Voila, no taxes!
If you haven't been doing this, the good news is you can still do it now. I got lazy and didn't file taxes for like 3 or 4 years. This would usually be fine, since your income is $0.00 - if you don't owe anything, the IRS generally won't bother you. (This might be why H&R Block said don't worry about it.) However, since we're sponsoring an immigrant, the IRS is going to get nosy about our income - so you've got to turn in your $0.00 tax returns for at least the past 3 years to prove that you never owed them anything.
Then you will fill out the i-864 with your $0.00 income and your mom will fill out her joint sponsor form with her income.
If you search these forums, you will find this exact question from many, many people in Japan who haven't been bothering to file their tax returns. The advice to them is always the same: you should turn in your $0.00 tax returns for the past 3 yrs in order to do the i-864. Just doing it is actually probably easier than stressing out about contacting H&R Block and worrying yourself sick over whether or not your letter of explanation will make NVC happy.
One tip: if you turn them in and at the same time mail in a request for transcripts of those years (here: http://www.irs.gov/Individuals/Get-Transcript), they will get done very quickly!
Good luck!