I understand this delay of a noun or adjective until after the verb, separating the modifier from what it modifies, to be very common in Latin. (But I am not an expert.) It often serves to emphasize what comes after the verb, though understanding which element receives the greater emphasis always calls upon understanding the sense of the words. Or it can just make a nice cadence by ending on the most important concept.
Here's a typical example, from the Venerable Bede (Historia Ecclesiastica, Cap. XIV):
…visum est fratribus triduanum jejunium agere et divinam suppliciter obsecrare clementiam ut misericordiam sibi dignaretur impendere et sive periclitantes hoc morbo a præsenti morte liberaret seu raptos e mundo a perpetua animæ damnatione servaret.
The brothers [at this monastery] thought to do a fast of three days and to humbly implore the divine mercifulness that they be deemed worthy to devote mercy to: that those in danger from the illness be delivered from immediate death, or that those already taken from the world be saved from perpetual damnation of their souls.
The adjective divinam leads the listener to expect a noun. By delaying the noun until after the verb, the noun gets added impact. The brothers aren't just appealing to God, they're appealing to a certain attribute of God: his gentleness.
Horace uses this trick to great effect at the end of Ode 3.18, which describes the biannual festival day for Faunus, when everyone takes the day off work and gets drunk.
gaudet invisam pepulisse fossor
ter pede terram.
Glad is the ditch-digger to have struck, three times with his feet, the hated ground.
"Three times with his feet" refers to a dance. The humor is in thinking that the ditch-digger's job is so odious that he has come to hate the ground so much that dancing on it is for him a sort of revenge. The grammar and word order set this up by starting with invisam (what is hated? you don't know yet), then introducing the ditch-digger, and only at the last word of the sentence providing the noun.
Now, looking at your example:
Dominus nos benedicat
et omni malo defendat
et ad vitam perducat æternam.
I understand the delayed æternam to emphasize that this is the goal, the endpoint, the ultimate that all the others lead to. Its breaking of the parallel structure further emphasizes it. In English we can get a similar effect like this:
…and may He guide us to life everlasting.