1. Inter may appear after its corresponding object.
- rex inter prīmōs cōnsisterat: he was mad a king among equals.
- mediōs inter hostēs Londinium perrēxit: he pressed on to London amid enemies (enemy ambushes.
2. Except (perhaps?) for metrical purposes, intra will always proceed it’s corresponding object.
- intrā trēs diēs: within three days
- intrā lūcem: before the day was done
The Essential AG: 435
Latin for Addicts 
Salve! I just found your blog on facebook and I really enjoy the detail. I would have to disagree, however, with your first premise about “inter.” In your first example, “inter primos”, “inter” clearly precedes its object “primos.” In the second example, only the adjective has jumped in front of the prepostion (which is fairly common, as in “magna cum laude”), the actual noun-object follows it. Hence “multos inter hostes.”
In Lewis & Short I find only one example of “inter” after its object, and that from Tacitus, “qui bellum et pacem inter dubitabant.” Tac. A. 12, 32, but the actual texts of Tacitus that I have found so far have the reading “qui bellum inter et pacem dubitabant”, where one object follows and the other object has been fronted as one would an adjective.
Thanks for your reply, Diane! I’ll hope that others find this comment and can draw their own conclusions. (Sorry for the late response, I haven’t done maintenance on this site in a while, but I do appreciate your feedback.)