Honestly, at this point I'm ready to throw in the towel and admit the odds are a hundred to one they'd ever need a rescue craft. Maybe a bit higher that missions will fail fatally--but some possible accidents will kill the crew instantly--there's a silver lining of a ghoulish sort!
Odds seem good nothing fatal will happen.
I'm still haunted by the idea that people sat down and did the same checks and came up with the same low odds for Apollo, and yet Apollo 13 still happened anyway. And thank God that Service Module was not the one picked for Apollo 8 or Lovell and the rest would have been dead right there. And not instantly but in a diabolically suspenseful and horrifying way at that.
However--I'm glad I worked out that it would take an entire Heavy launch just to get a single freaking Apollo LEM there (using storable propellants that is--it could be twice as much I suppose if we could only use hydrogen-oxygen...which requires a new facility in orbit to keep reliquefying the hydrogen). Even if we could double it, would as much as six tonnes of supplies be enough to keep 4 astronauts alive through the months, two or more Lunar nights, it would take to get their second LEM to them?
The killer need I suspect is power. We have yet to be told just what sort of power levels it would take to keep a habitat above freezing temperatures in the Lunar night, but I imagine it is not trivial. The Rescue Columbia timeline was certainly on my mind, and we quickly concluded, last late winter/early spring when that was being written, that after CO2 absorbers the next thing Columbia crew would need to survive would be power. I figured getting them oxygen and hydrogen for the fuel cells might be doable, and these provide the benefit of not only keeping the essential lights and fans running but also drinking water. The only alternative I saw to sending fuel cell reactant (and if necessary the fuel cell itsel) to power the habitat at night was some kind of radionuclide generator. Either option--keeping fuel cell reactant or any kind of nuke power--seemed terrible from the point of view of keeping it stored in orbit for years just in case it might be needed. And when we've sent the fuel cell reactants, if we somehow could--what room is left in the cargo for the other vital necessities they'd need?
Mind, I had yet other notions, but with payloads measuring less than 10 tonnes I had to admit no matter what, we'd be licked. They just can't hold out long enough.
The mission profile does allow a lot of redundancy as is; accidents that wreck one craft or the other might still let the crew get home. That's an advantage of parking at a Lagrange point after all--just a little push and you can fall back to Earth, provided the CM heat shield is still intact.
Which is why my first thought was to have a rescue craft based there; from there it can go anywhere. Of course what I really wanted parked there would be a depot.
Given that the program can't, for political reasons, propose to develop any permanent infrastructure I have to admit the authors seem to have come up with the best answers, and that further safeties just cost too much.