While a good plan in theory, having an unmanned craft dropped off in advance of the trip means subjecting that craft to an average -55 Celsius at night and 20 Celsius during the day. And there is the low thermal inertia which forms the winds combined with the planet wide dust storm, the volcano, solar wind and cosmic radiation – basically not a place where I would want to fly off in a spacecraft that had been lying around. In order to be confident in the collection of the proper collection of fuel the craft would need to land more than 6 months in advance so that the astronauts would not leave for Mars until they knew they had fuel to get home. Of course the craft would still be sitting there aging in that unforgiving environment. Perhaps that part could be dealt with by deigning it with a shell that is simply blown off to reveal the craft when the time comes. And another possibility is to have the craft be kept in orbit while the driller comes down to the planet to obtain the methane. If the ship came with enough fuel to get up in orbit over Mars and they were able to link up to the return craft in orbit, that might work.
It is therefore important to consider the possibility that maybe the return would not be possible and it would be necessary to have not only the return trip panned but if necessary the staying and colonizing. It has already been established that SiO2 is a fundamental building block of all stony planets. This along with all of the other ingredients in the Mars habitat means that should the astronauts end up staying, a simple procedure via a photo-chemical procedure or electrolysis process could give them oxygen. Water already exists in the form of ice beneath the surface and the drilling device was accessing it to manufacture methane. Of course that would leave food and there aren’t exactly herds of wild animals to hunt.
The advance vehicles: the return craft and drilling device, could also include a hydroponic set-up. With the many advances in robotics, why not send one along that could set up a hydroponic agricultural sphere. Perhaps plan the mission as sending 5-10 people and maybe have four stay behind and the remainder attempt the return trip.
The apparent difficulty of the entire trip rests in the return trip. NASA has sent many rovers and in recent history, very successfully so obviously we have that part down. It all comes down to the return trip, so there may be simple answer, don’t return. Or at least plan for both staying and leaving. NASA has the right idea with regard forming the Moon colony with the help of a 3-D printer. While the Mars surface is not filled with the dust that the moon has, it could surely be used in a similar manner and again the advance equipment sent could include this and a robotic presence could help all this be prepared for the soon to arrive astronauts. And it could all be recorded for a movie, A Return From Mars or Not.