For the first part, I meant you need to check the box that the TAXPAYER (parents) are not a dependent of somebody else.

For the second part, you are required to look up the numbers.  If the Marketplace does not issue Advance credit, they don't necessarily ask the questions they need to calculate column B.  You also would need to enter a different number in column B if something changes (such as your Family Size on the tax return is different than what you told the Marketplace, or if you did not report such a change to the Marketplace).

It is a fairly common thing to look up the numbers.  I'm unsure if the taxpayer will receive a IRS notice asking about it or not, but the IRS doesn't read attachments so there is no point in doing that.  Hypothetically the IRS has the ability to look up those numbers too, so hypothetically they would not need to ask you about the numbers you entered.  Whether or not they are actually set up to do that or not, I'm not sure.