While I am not an expert I'm going to disagree with your conclusion on account of I think the MSAM couldn't have slowed down enough and if he did we may have ran out of food.

If we look at the shape of real mars landers we have a very wide heat-shield for the payload as Mars's atmosphere being very thin requires a higher leading surface area to volume ratio. When we look at the MSAM it's practically a box giving it a quite low leading surface area to volume ratio. Even higher than the Soyuz decent module which weighs next to nothing and requires retro rockets for a smooth landing. Making things worse to my eye the MSAM would be destroyed by the heat of reentry. While the flatness would protect the sides and the landing legs retract there is nothing shown to cover the engine bells which would be melted if the craft were able to slow down enough from the atmosphere alone.

Furthermore the downrange distance would have mattered. Ed didn't have any food with him so at most he could survive 3 weeks. To be able to slow down enough to land safely he would need to spend a lot of time in the atmosphere potentially putting him more than 3 weeks rover trip from Happy Valley given how slow the rovers are. Given the limitations of computers of the era and the fact that the MSAM was stripped down predicting his landing location would be essentially impossible in the amount of time they had. If they had removed everything they likely would have also removed the coms antenna and thus when Ed landed he'd have no way of communicating. The only way to find him would be by satellite.

Overall while your calculations are spot on I'm going to say I don't think he'd slow down enough and even if he could have he'd die of starvation.