now looking closely, the extra confusion is that the 1st screenshot shows a "+1" for an additional person, but the 2nd screenshot doesn't...
I think the +1 is "crosscountry and one other operator" rather than representing an extra person.
2. the rationale for the £512 difference between University and New Street, for the 6 minute journey that actually connects with the train from New Street
Two key things to understand.
1. Rail ticketing sites were traditionally built around selling the passenger ONE ticket for their whole journey wherever possible. More recently "splittling" sites have appeared which try to find cheaper combinations of tickets, but such sites are still the exception not the rule.
2. Advance tickets can only be sold against reservable trains if quota is available.
It used to be that most local services were not reservable, but in recent times more and more have been made reservable as local operators have started to offer advance tickets.
If the quota "sells out" for the local service, or if no suitable quota was allocated in the first place, then the reailers can't sell an advance ticket against that itinerary. A retailer that does splitting may offer a split ticket, but most retailers will simply fall back to selling the user flexible tickets.