It looks like there was some previous discussion on this and the 2 episodes were combined, but this is not consistent with how NBC has them listed.

https://www.themoviedb.org/tv/954-the-rockford-files/discuss/5ae094c1c3a3683dcf01568b

The important thing is not how NBC list this now, but how it was originally broadcasted.

That's fair, but I wasn't able to find any evidence of them being a single episode when originally broadcast. The only reference to this is from 'Steve' in the other thread I linked.

IMHO, 2 episodes broadcast on the same day are still 2 episodes.

Other sources also have these split.

https://www.tvguide.com/tvshows/the-rockford-files/episodes-season-1/1030260610/

https://en.wikipedia.org/wiki/List_of_The_Rockford_Files_episodes#Season_1_(1974–75)

https://tv.apple.com/us/show/the-rockford-files/umc.cmc.xi9qkywkfmt0aea0viqavys6

http://epguides.com/RockfordFiles/

If you look at this page (linked from one of your links), you will read "Originally shown as a 90-minute episode; later re-edited (with much repeated and redundant footage) into two one-hour episodes."

Interesting.

However, this means there is ~10 min of official episode(s) which are not covered by TMDB, since the single episode listing would only cover 90 min of footage.

Would an Episode Group make sense here?

An episode group can't be used here. With an episode group, you can only reorder existing episodes. You can't split an episode in 2 parts.