I would think that now you know abut the rust you should repair it, since it's already off the road, and then tell Shannons that it has had rust repairs, which may even increase the agreed value of the car. You've already told them it had some rust bubbles, so they would have taken that into account when valuing the car. They are not silly, and would have realised that if it has rust bubbles showing then it is likely to have rust underneath.
However, now that you know that it has rust, if you just put the car back on the road and then have a crash, if it can be shown that the rust has contributed in any way to the crash, or to the severity of the damaged caused by the crash, and this conversation was to become known to them and they could show that you had previous knowledge of the rust and did nothing about it, and did not notify them, then you would not have a legal leg to stand on.
You do have a duty of care to disclose anything you know, or could be reasonably expected to know, about that could be taken into account for the insurance value or premium. As you said, you restored the car, so if they found there was rust in the car that you should have known about, even if you say you didn't, then they would be able to avoid any payout.
So, I would suggest the simplest thing in the long term, though it means a lot more work in the short term, is to fix the rust.
My 5c (inflation) worth.
Watto.
