One Exodus scroll, 4QpaleoExod-m, follows the expanded Samaritan-type text, not the Masoretic. Isn’t that a different Exodus at Qumran?

Correct, and it is a good example of the textual plurality the Scrolls reveal: in the last centuries BC several text forms circulated side by side. The expansions in the Samaritan-type Exodus are mostly harmonizing additions that repeat material from elsewhere in the Pentateuch; they do not introduce new content or change the narrative. Seeing that form beside the proto-Masoretic one is exactly how scholars learned which readings are older. The claim is not that only one Exodus existed but that the one that became standard was faithfully preserved.