kinda difficult to respond. I would struggle with a solo player, but I’m looking forward to my daughter growing into a an age to play MG and other RPGs. So, I’ll try to think of how I would present MG in case she is a solo player.
Playing a second character, as though the patrol were two Guard members, I suspect generates some trouble with separating GM knowledge and Player knowledge. This may be particularly related to having a strong inclination about how to face obstacles with a solution that highlights the best skills and easiest attempt. So, I would suggest an alternative: have the solo player develop a Patrol Guard; have the GM develop a recurring NPC (an NPC who is more like a Guard with skills, traits, wises, BIGs, but no Fate/Persona): the recurring NPC can join for some missions, but not others; have the GM develop the relationships–particularly Mentor and Artisan–similar to the recurring NPC so they could join for missions from time-to-time or show up during a session to join for an obstacle only (in this case, the Friend and Enemy are less of joining in, and more of getting tangled in); have GM develop a cameo NPC for each mission who really wants to be involved in the outcome, so has the skills, traits, wises and gear to assist, but probably couldn’t take over from the Guard.
In the above case, the solo player has a Patrol Guard who can handle independent missions. Having another NPC that joins, but doesn’t become a reliable resource (e.g. doesn’t stick around through every obstacle, doesn’t join for every mission, doesn’t have answers most of the time), offers assistance and support for a Patrol Guard to take on juicy mission assignments, but doesn’t undermine the autonomy of playing solo and having the GMPC always giving hints about what will really succeed. This can allow the Player to attempt unexpected solutions for obstacles without feeling the ‘supervisor’ GMPC will make corrections. That adds risk, and reduces railroad.
Also, the different NPCs can be used for specifically thematic missions. The Mentor can join the former pupil for a mission to re-teach a lesson, to coach development of a new skill or wise, to illustrate development of old skills or wises, to counteract an overwhelming trait, to contradict a strong opinion, etc. The Artisan can join for missions about new markets, new materials, new merchants, old traditions, old trails, old scoundrels, flirting with social mice, fighting with rascal mice, selling to greedy mice, etc. Using those NPCs with a special theme can develop the campaign stories (e.g. “Oh, here comes John the weaver. We’re off to find rare silkworms again, right?”). Having an NPC who has skills, traits, wises, and gear similar to a Guard, but is not a Guard member can illustrate rivals, partners, or a special mission beyond normal boundaries of jurisdiction (e.g. “Well, good to see Constable Regina along. I guess we’ll be headed into the hinterland to arrest a Wolfepointe chieftain for this mission.”). The cameo NPC helps bring things into a local perspective, these are mice who are intended to personify their town/city for the Player; this is a little like playing out Ron Weasley–memorable, and shows a clear view of Weasley family traditions in most scenes–where players can quickly see a quirky settlement feature that is played out through a stereotype, caricature, or prejudice. That might sound a little rude or lacking political correctness, but try to think about it like a show or film. When you don’t have much time to showcase a settlement, using a stereotype, caricature, or prejudice can very quickly highlight a few features of place, people, and purpose.
So, regarding obstacles for a solo player, I would be cautious about the choice of animals and weather, but not too worried about wilderness and mice. In the case of animals, you can choose animals with lower Nature ratings that still bear similarity to the animals you initially think of. So, don’t use badger, use skunk; don’t use raccoon, use marten; don’t use bullfrog, use toad (or frog); don’t use snapping turtle, use turtle, etc. There are many ways to downgrade the choice of animal, yet still maintain a challenging foe for a solo Guard to face in fight, chase, or science! In the case of animals, also look closely at how the animals impact the lives of mice in manners other than predator-prey relationships; having to manage a flock of geese who came by while migrating, is a huge task for a solo player, but doesn’t require they fight the geese. Those geese might be a yearly distress, and helping mice with an early harvest might be just as good as rallying them to mob the geese in an ill-fated attempt to chase. Weather is another realm in which you can easily downgrade for a lesser event. So, don’t use Spring Rains or Spring Storms, use Spring Rains; there are other ways to ease the effects of weather in other seasons too, but it will take some looking at. There are two ways to reduce the impact of weather in the narrative content also, when the weather is impacting distance trekking or passing over an area, that helps ease the hazard for mice. If the settlement life is fairly good, but the trail life is overbearing with heat waves, you can easily cause fatigue of trekking to cause both Thirsty, and Tired, but once the Guard arrives in a town, someone offers water–the player is left trying to handle the Tired Condition, but gets a free assist for the Thirsty Condition; this might seem weak on the GM part, but I will disagree. If you send a Guard trekking during the first obstacle, and they happen to fail the Pathfinder test (or whichever test is given), then GM can offer success with Hungry/Thirsty, and Tired. Seeing the problem of two Conditions, the player may quite readily seek to gain a Check during the next obstacle to attempt recovery for both. Offering the free recovery from Hungry/Thirsty now opens the player Check for something else. It can be for completing the mission, having a personal hobby, or whatever.
Now, in contrast to the above, I don’t think that Wilderness and Mice obstacles need to be softened for a solo player. In fact, I think those should be about the same as ever. The difference I would have here is to offer more complex tests; while for a patrol I might offer more simple tests. So, in the wilderness, the solo Guard must test Pathfinder and Survivalist, while the patrol might only test one with the other serving as Helper die. Dealing with mice might lead to Persuader and Haggler while the patrol might only need to Haggle while a Persuader offers Helper die. That’s dependent on GM style. But, My line of thinking is that the solo Guard doesn’t have a patrol to help cover all the tasks of wilderness living or sorting through settlement living; there are just more issues and chores to be completed and the same Guard mouse has to be actively getting each thing done, and can’t do everything at once. So, the complexity helps to illustrate the one-thing-after-another nature of doing things alone, offers more chances for something to go wrong, and allows for a player to gain more checks for recovery or other matters. Certainly, having an NPC along to assist can be a benefit.