OOC Thread-Pre game
First on the subject of mobile nodes: the only canon version i am aware of is the Folding Gate of Armaghast on pages 52-53 of Forged by Dragon's Fire book, revised edition. this is a 10 point wonder.
Second, Umbral travel is mostly spirit magic yes, but not exclusively. Mind 4 allows travel into the Astral Umbra. In fact it almost requires Mind magic to reach the higher states of the Astral Umbra, however rare exceptions exist. This method is basically astral projection unless you use other spheres to project your body there.
To travel to the Dark Umbra/Shadowlands of Wraith you need Entropy 4/Spirit 3/Life 2 for yourself to enter bodily, Entropy 5/Spirit 4/Life 3 for others.
There are rotes that don't involve spirit magic to enter the Digital Web, but these usually require normal or magical computers/VR tech and all involve correspondence magic.
Now it IS possible to get from one area of the umbra to another (either from one umbra layer to the next or to an entirely different umbra). These can involve literal travel through the umbra or using correspondence magic to get there. The most universal of these paths is the realm of the Axis Mundi. However other ways exist, such as through dream realms which can touch all three layers. Other portals always exist, but that is often a matter of finding them and knowing some cosmology is VERY useful for that if you are so inclined.
Anything beyond the Horizon is not yet attainable by the group unless they have items/allies/patrons with the ability to transport and/or survive out there. A full ethership as is being discussed in another thread is one such possibility.
The above is NOT an exhaustive list of what is possible, canon or otherwise.
Having said all of the above, I am friendly to making umbral travel more accessible, and I like to award player sphere creativity. there is ALWAYS another way to travel, just a matter if you have the right mindset and spheres at your disposal. One thing about how spirit travel works is that it is very easy for one member of the party being able to travel to a part of some umbral layer that nobody else can. So ideas that get around this problem I am supportive of, long as some logic is there. and logic to mages is inherently subjective, remember that.
third: non mages entering/traveling the umbra. yes this is possible, but it varies a lot. for vampires, there are various methods. necromancy has more than one way to enter the shadowlands (depending on edition and game era). High level Auspex and maybe a few other odd disciplines allow limited forms of astral projection. going into the middle umbra/spirit wilds aka where the fera mostly go is much harder for vamps but not strictly impossible. mostly only wyrm spirits would like them, even then not always.
Kuei-jin can most certainly enter the various umbra with multiple methods and do so with some regularity. in fact interacting with spirits is part of what they do.
Sorcerers can enter the umbra on their own, but it is rare and powerful abilities that allow it. the main way canon wise is oneiromancy 6, allowing physical entry to the dream realm, and thus if careful to other parts of the umbra. I would allow lower dot methods of entering the various umbra physically, but they would be high level rituals involving dots in multiple paths. powerful enchantment or alchemy could work, but i would require at least 5 dots for anything remotely permanent, with probably drawbacks or limitations. for more temporary use of umbral travel, lower dots are just fine.
fourth, for sphere crossovers, well sometimes having the same sphere is a great boon. it allows for helping in ritual castings, the more similar the paradigm the better, at least for the effect involved. and lots of prime magic is never a bad thing. every mage needs a bit of prime past a certain point of what they can do. it is the great enabler of magic.