Green car transfer/ticketing on regular lines, am I missing something?

Hi everyone, so I recently got Mobile SUICA after upgrading my phone and I want to know how something works.

I often make trips from Abiko to Utsunomiya on regular trains (the Joban rapid line and the Utsunomiya line) and I pay for a green car ticket using 600 JRE points that I've built up.

Until now, I've been using a physical SUICA which required me to 'apply' for the ticket the day before I travel, go to the platform machine by the green car boarding area, tap my card, hit 'Abiko' and 'Utsunomiya' on the map and then I'm good to go.

After downloading Mobile Suica I learned that green car tickets are available instantly on the app, but when I try to search 'Abiko' to 'Utsunomiya' nothing comes up.

The internet is telling me that I can actually just buy a 600 JRE point green car ticket between any two destinations to bypass the app's system and just ride as normal… but I'm worried about the transfer and things going wrong.

It seems strange that the system would permit someone to buy a green car ticket to a certain destination, and then let them get off somewhere else hundreds of kilometres away after using a completely different line…

I've ridden the green car on regular trains about 10-15 times and I've found that around 50% of the time, either I've done something wrong or the system gives me a fat 'no' and I have to speak to the staff to get it sorted. Sometimes I end up paying more money to the train guard. I'd like to avoid that by understanding it all better. Can anyone help me out?

by ClemFandango6000