The CR has access to Deuterium in Coronado and maintains an own mining fleet (CMI-tagged ships), for many years and with quite some RP behind it.
So the supply with most war-critical materials is likely to be more or less reliable. The same goes for the other civilan manufacturing and shipping companies, that are part of the faction (CDI, ISE,...).
Considering that IMG (at least in the Taus) has not been a reliable ally ever since Gallia popped up, which made cooperation with them, at least in this region more or less impossible, one might expect that trade with them has already fallen to a relatively low level for a longer time now. Calling IMG one of the closest allies is not very accurate, in general one could state that the importance of the alliance with the IMG has decreased, while the one with Bretonia has become more and more important. So the overall effects would not be too bad.
While there might indeed be shortages to certain materials due to this turn of events, the inRP consequences will likely not turn out totally catastrophic.
From an inRP standpoint the decision for the CR is relatively simple. As long as Gallia is focussing on Bretonia, we are relatively safe and have a chance to fight back. If Bretonia dies (which it certainly would, if there is no way to repair ships), then we are next.
We have provided two major solutions to solve this issue without 'going to war'.
1. Bretonia can use parts of Sabah shipyard instead -> This is not really an option, because there is no secure passage to Coronado
2. CR, in agreement with IMG, dispatches marines and declares that Aland is under CR control, then allows Bretonian ships there; Once the war is over IMG gets everything back, if something goes wrong, they can blame us and get around trouble with Gallia -> This was more or less ignored due to, what we consider to be a bit of an OORP conflict of interest on the IMG side
Considering that neither option proved to work out, backing up Bretonia was the only logical option that ensures the survival of CR.