I wouldn't say it's deliberately aimed at splitting, a lot of train operators do it out of a (perhaps misguided) attempt to
protect seats for those doing long-distance travel by avoiding wasting it on local journeys - e.g. today I did MAN->RUG and was able to book a seat even at the last-minute on a 9-car Pendo; that's a good thing. The nature of how the RARS interface has been designed though, means that the overall passenger origin and destination is not available when this logic is applied and the passenger is using multiple tickets to make their journey, so they'll be treated like a passenger doing much shorter journeys.
The irony of course is that there would be far fewer problems with seats getting taken up unnecessarily if the services weren't incorrectly marked as "Compulsory Reservations" in the timetable.
The problem with bucket nesting is that it causes real problems with caching too, because suddenly it's not just "there's availability 5 between station 1 and station 2", it's "there's availability 3 for fare A between station 1 and station 2
together with fare B between station 2 and station 3" and it's not remotely practical to retrieve that information to put in a cache for possible combinations.
This is one of the main drivers behind musical chairs on CrossCountry services as well. CrossCountry try and automatically allocate certain passengers travelling between certain station pairs (which may be indicative of having luggage) to seats near luggage racks. It's a clever thing to try and do, but the minute you have a split somewhere, it can mean it becomes effectively impossible to get the same seat throughout, because of the way the seats have been artificially "segmented".
Side issue: is there a way of diverting notifications to a third party when tickets are bought for someone else?
The booking can be moved to someone else's account on request and will then appear in their app/send
them notifications, but that's all the backend supports today. Various planned pieces of work to improve the experience in that area.