pwyg- If you think using a 1" steerer will prevent main frame damage in a frontal impact I think you will find yourself wrong. The Schwinns used a .833" stem, not steerer. As much of what we know of the US made Schwinn product design was based on left over WWII tooling (some paid for by the government) and by the grades of steels commonly available back then. Schwinn did do a good thing and choose a steerer with a thicker wall to help off set the low strength grade of steels they usually used.
As to converting horizontally slotted drop outs to a through axle without replacing the drop outs- Sure one could. The adaptor/conversion part would likely be specific to each brand and model of drop out and also need to accommodate the various stay attachment shapes and such. Fine to one off if your time is not expensive. Please keep us in the loop of your efforts on this. Most here enjoy seeing others' cool creations, even if some will also shake their heads at the results
Andy