B2 extension for fiancee


I am currently a student on an F1 visa and my fiancee is in the US on a B2 visa her stay was authorized until July.
We have filed the I 539 about 40 days ago to get an extension and plan to get married eventually so she can stay on a F2.
We recently learned that the processing time is so long, that we might not get a decision before her authorization ends.
When we initially filed the I 539 it was recommended that we file it at least 45 days before the authorization ends, which we did ( more than 100 days).
Now, I am surprised to learn that the average wait time is 6 to 8 months, wich doesn’t really make any sense considering that the B2 is initially only valid for 6 months. So you would have to file for the extension before even entering. All of this is, of course, made worse by the current worldwide situation.
At this point, i feel like our only options are for her to overstay her B2 or to take the gamble and try to reenter through Canada and pray for a nice agent ( we live very close to the border). Non of which seem like good options.

Is it correct that she is allowed to lawfully reside in the US while the decision is pending even if the I94 authorization has expired?

I would greatly appreciate any advice on the matter.

Yes - you can stay legally while a petition is pending approval. You are not at fault for the delay. Make sure that you keep all receipts and correspondence safe, your application timeline is evidence