The service concern is a very valid one. Here's my perspective.
Engine:
BMW have over 360 dealerships in the USA, there are countless more specialists, the engine in the Grenadier is a BMW, in a pinch you could even take it to a Toyota dealership for service, if theres a new Supra on the forecourt then you'll be fine, Toyota have 1270 locations.
Transmission:
The ZF gearbox in the Grenadier is used by a lot of manufacturers, Alfa Romeo, Aston Martin, Audi, Bentley, BMW, Chrysler, Dodge, Great Wall Motors, Haval, IVECO, Jaguar, JEEP, JMC, Lamborghini, Land Rover, MAN, Maserati, Morgan, Porsche, Ram Trucks, Rolls-Royce, Toyota, Vin Fast and Volkswagen. So from this list we probably have a service facility for the transmission within a few miles of every town in North America thats before the independent transmission shops. AAMCO have over 600 locations alone.
Axles:
Let's say this is a hyper specific axle that isn't serviced in North America. But it's a solid axle made by a heavy equipment manufacturer. With the correct manuals, any competent 4WD mechanic, farm machinery facility (if they let you) or jeep mechanic should be able to service these. Transfer case etc would fall under the same I'm thinking.
General maintenance:
We have the EU to thank for the right to repair.
"Access to vehicle repair and maintenance information
Easy and clear access to information on vehicle repair and maintenance (RMI) is key to guaranteeing free competition on the vehicle aftermarket. Manufacturers must ensure that independent operators have easy, restriction-free, and standardised access to information on the repair and maintenance of vehicles. Discrimination with respect to authorized dealers and repair workshops is not allowed."
So there will be maintenance manuals available in Europe at some point, the NA Grenadier is the same as the European vehicle in almost all regards.
I think I've covered the most complicated parts of the vehicle, yes there are very few INEOS dealerships and designated service centers, but you don't have to use them for servicing and routine maintenance. Unless I'm mistaken, there are laws in the USA specifically to address this and protect the consumer. For warranty work, INEOS can nominate a repair facility to perform the work required if they wish.
To finish my rambling on this Sunday morning, I'm lucky that I'm in LA* if I was in a state without a dealership then I'd be miffed by the lack of convenience however I'd probably enjoy the road trip home because if I'm buying this vehicle then I'm going to enjoy driving it.
All in all I'm not that shocked or worried or concerned about the dealership coverage, warranty issues or maintenance and servicing, provided parts availability is ensured. This is just me though and we make decisions, especially very expensive ones (which for most of us this is), based on all the information at hand.
*(yes my Grenadier if I purchase will be used to go on the school run, grocery store and to commute as well as all the other day to day things that I need a vehicle for including recreational off-roading.)