I would just ignore that. They know you have had your interview, and that message has no meaning. You may see it change to READY and even REFUSED again, before ISSUED.
Totally irrelevant.
Even with one meeting, the OP will have the petition approved. The bar for that is very, very low. The issue is visa approval by the consulate, which you obviously have not experienced yet.
Where did you get the idea that invitation letters are a thing for US B visas? They are not. Nor is there any type of financial sponsorship.
The consulate does not care why they want to come to the US. They want to know why they will return to India. Your in-laws will have to demonstrate strong ties via the content of their online application. Any documents brought to the interview are generally not considered.
You cannot pay to have that type of visa expedited.
And they will not expedite just because you are tired of waiting. There are people who have been waiting for much longer than you.