Re: Just add SPICE
It may be easy to calculate a spacecraft position far into the future from a SPICE kernel, but that doesn't mean that the calculated position will be accurate. SPICE is just a data format and a set of subroutines to interpret that data. It's not a panacea for the fundamental difficulty of accurately modelling the orbit of spacecraft around Mars.
I'm guessing that the major stumbling blocks are uncertainties in the gravity field of the planet and the atmospheric drag -- remember that MAVEN is actually sampling the Martian upper atmosphere. Both of these uncertainties severely limit the accuracy of a long-term ephemeris of a spacecraft in an orbit like MAVEN's.