It's great that you're being so supportive, but there's a limit to what you can do to help. Thru-hiking is all about self-reliance, so your mother really has to develop her own skills and mental resilience.
It sounds as though you're already on top of your main role, which is to hold the fort in terms of managing her finances and resupply.
To put your mind at rest, she won't be in "natural solitary for 6 months". The AT is a popular trail and highly sociable. If she wants company she won't have any trouble finding it. With the exception of the 100 Mile Wilderness, it's also mainly near to population centres, so she can be in touch with family and friends on a regular basis. And as you seem so concerned for her safety, it might be a good idea for her to carry a Garmin inReach Satellite Communicator so you can track her progress and exchange messages when there's no cellphone reception. It also acts as an emergency beacon if she ever needs help from Search and Rescue - this is a big deal for solo hikers and will greatly enhance her safety.
As you say, there's no shortage of online videos and advice, so she should be well briefed. It's the world's most vlogged trail! Though from what you say, her experience is quite limited for such a big project. From what I've seen, many US thru-hikers set out under-prepared in terms of basic back-country skills. For your own peace of mind, you might want to double-check that she is up to speed in the key areas that people tend to neglect:
Basic navigation skills: the AT is a well-marked trail, but you still need to develop a sound grounding in navigation with map and compass. You may never need it but if you do it can be a life-safer - just a few years back there was a fatality when a lady missed the trail and become hopelessly lost. Too many people these days rely solely on their cellphone GPS. I was recently on a thread with some of the most experience thru-hikers on the planet, and all agreed that this is a terrible idea - she should be carrying map and compass and know how to use them. I recommend Lyle Brotherton's excellent Ultimate Navigation Manual as a resource, or she could do a course.
The ability to deal with wet-cold: the AT is prone to some truly terrible weather, particularly in the White Mountains. There have been many incidents where people suffer from exposure and get into serious trouble. There's a skill to walking safely in bad weather in terms of equipment selection, management of pace, judging the conditions, selecting escape routes, erecting your shelter in a storm etc etc. There's no substitute for experience - encourage your mum to get out in bad weather over the winter to test her setup and gain confidence in her judgement. Obviously, this should be done in safe situations where it's easy to bail out. If there's a local mountaineering club she could hook up with them and learn from the old-timers.
Walking ergonomics: very few people think about this. But there is a real skill to walking efficiently and without injury, especially up and down steep hills. And the AT is pretty much all steep hills! I learned much of what I know from watching the old guides in the Alps, but also from research and experimentation. Experienced walkers can quickly spot other seasoned hikers on the trail because they tend to walk more efficiently. It's too big a topic to go into here, but encourage her to research it, or go on a forum like BackpackingLight.com and ask for advice. On such a long walk this can make the difference between success and failure. Also, most people who carry walking poles have little idea how to use them - there's a technique and she should learn it. This helps preserve the knees of those of us past the first flush of youth...
So as I say, these are the skills that I feel aren't given enough attention by the typical newbie thru-hiker. I hope something here is helpful. I won't duplicate the topics that are covered well on YouTube except to repeat that as a newbie she should start slow and work up her mileage gradually. Many people start too fast, get injured or demoralised and have to bail. It's a marathon not a sprint - for the first 6 weeks or so she should stop well before she feels she needs to. She'll make up the time later on when she's trail-hardened.
So I wish her all the best - it should be a great adventure. And although I've highlighted the dangers, just hold onto the thought that the great, great majority of hikers come through the experience safe and sound, with their lives much enriched.