Hi Brian!
Good observations, as always. I just looked at a photo of a D-360, which would have been the only drop-down model prior to the 258, and I saw no toe switch, so you're probably correct that the 258 can be tagged as the first. Going by memory, other models with the toe switch were the 259 (258 with a chrome hood), the 238s, 248 Commercial, household 240s (with the toe rocker switch), and the 228 with toe switch prior to the Dial-A-Nap. It may be that some private-labeled uprights in the '60s had them, as well.
I wish Eureka sales sheets were more plentiful like Hoover's seem to be. Time and again, I've seen people posting them for Convertibles, which often answer questions about models' features. The McLean Historical Society seemingly does not have them in its Eureka records. I have heard nothing back from them for about a year now.
You make a good point regarding the Eureka F & G's aero-dynamic cavity in comparison to the Hoover Convertible's. Maybe Vaclab or someone else will do a comparison test sometime to verify. It would be even better if different tests could be done with different brush rolls, Hoover types C & A bags, boxtops bags and Hoover's 13-quart bags to see how those features contributed to the overall performances of Eurekas and Hoovers on a variety of carpeting styles.
It's really bugging me that I haven't found out what model that gray Eureka is that I'd taken for granted was the 258-A. My guess is that it's a 258-BA, -BG, or another double letter that often occurred when the only change was the color. Whatever it is, it must have had a short run, because I have found no information for it so far.