Rats are not specifically attracted to PEX, but they do have to "seek out opportunities to" chew on things all the time. Their incisor teeth never stop growing, so if left unchecked they would grow so long that the rats would no longer be able to open their mouths and starve to death. So they are compelled to keep them worn down by biting on things all of ...
If 1/2 PEX would have been large enough to do the runs but the contractor used 3/4 instead, then:
he (you?) paid the higher per-foot cost on the larger tube and paid for each adapter
the tube holds more water, so if this is a hot water run, it'll take comparatively longer for the hot water to arrive at the fixture.
If the runs really do merit the use of 3/...
As Michael Karas' answer said, cost will be minimal and it will be easier just to run a new pipe.
Additionally, do you use the outdoor sink in the winter? Insulated or not, you should probably winterize that plumbing if it's susceptible to freezing.
Since you say there is access on both ends it seems that the best course of action is to just pull in some new PEX line with insulation sleeve installed. The cost of materials should not be so much as to break the bank account and it would be far far easier than trying to save what is tunneled into that narrow area.