do case endings fall into disuse because folks are using fixed word order and prepositions, or do they start using fixed word order and prepositions because of their case endings going away

Probably it's a mixture of both forces at work.

For eta: If we had a unique case ending for the recipient of an action and another for the instrument used to accomplish something we could, for example, say "I hit manum swordo", "hit I swordo manum" or even "swordo hit manum I" or any other ordering of those words without needing any prepositions to indicate the the sword was the instrument and the man was the one hit by it. But if the dative -um ending and the instrumental -o ending, for whatever reason, became both pronounced the same, maybe -u, then some other method of distinguishing between the dative and the instrumental would be necessary. Maybe word order would do it and you could say "I hit manu swordu' or maybe you would use a preposition for one or both of the cases saying something like "hit I (with) swordu (at) manu" where you would use at least one of the prepositions.


Pipped by the master. But note: In Latin the instrumental and the ablative had already conflated, giving us what is normally called the ablative of means. Typically the instrumental took no preposition but any ablative use normally did take a preposition. In English it was the dative and instrumental that conflated.