The Aussies (and possibly Indians) will not have to stand in line behind the USN - they will get jets in parallel. Building another 6 737s in a given year is not exactly a big deal in Renton... from that point on it's a matter of capacity in the modification line.
The program's health right now is mainly a matter of software, which is indeed a big deal but (on the bright side) the Navy and Boeing know that this is where the risks are, and otherwise there is not much new in terms of electronics; it's either OTS, directly borrowed from the P-3 or upgraded P-3 kit.
It's even possible that if the RAF just wanted ASW/ASUW they could get earlier IOC than the USN, but off the top of my head I don't know what extras (SLAMs, ESM capability &c) are included in IOC.