I thought it was mentioned up-thread that they won't need them all until after some timetable improvements that won't be ready for this year.
It is true that the full timetable improvements won't be available until next year, however that doesn't preclude the use of four units this year if they're available.
the max unit length is six cars for both Oban and Fort William, and currently most Oban runs are four cars. You'd need to uncouple and leave a 153 at Oban for a while, but it's definitley possible.
in a similar vein, it's also possible to use a 153 on the Mallaig to fort bill sections, and leave it at fort hill for the next return run.
This is all very unlikely, however it is definitley technically possible to cheese it, provided several units stay away from cockerhill overnight at either Oban or fort William.
Until the Oban and Mallaig services on the WHL are separated, which will require a recast timetable (not happening this year), the Class 153 sets will only be on the 10:34 and 16:36 ex-Queen St and the 14:41 and 20:37 ex-Oban.
This is the current assumption yes, needing 3/5 units to be ready (2 in service each day) but if you really wanted to it would be possible to utilise the other two.
A bigger question is the whereabouts of the other three 153s, which won't be ready for a while if the current timescale continues.