So having read this thread for a day but without time to compose a reply I now can. While the photos are not conclusive, even a first hand examine can be doubtful initially, I do see a line running through the paint and rust area. I'd be willing to call it a possible crack. But there's a way to know for sure without resorting to sand blasting of more paint removal. That's to see if any liquid can seep through or not.

here's what I would do. First strip out the BB, cranks, post and such to have unrestrained access. Then Clean the outside as well as I could with solvent. Position the frame so that the "crack' was at the lowest point of the ST. Now add a thin liquid to the ST so a puddle sits on the inside at the "crack". Wrap a piece of paper around the ST at the "crack" wait and see if any liquid stains the paper. I'd consider using a penetrating oil. If there's seepage then there's a crack.

Safe, simple, no special tools and can be done at home in the living room if needed. Best is that if there's no crack the further rust prevention step has already begun. Andy