After all these years I finally got to and finished the Gaiden Route. Extreme spoilers ahead:
>Monaty, swallowing her pride(?) as a guardian beast, reconciles the fact she's powerless to help Magna up from his slump after the events of chapter 25. So she calls on her former master.
>It's both funny and cool that you enlist the help of SN1's protags to help solve your own issue. Although it's revealed that Mergitos' demon armies have been wandering the outskirts of サイジェント as well - effectively involving The Linker. I chose Natsumi and Claret.
>Everyone meets everyone! This lead to some funny moments. Forte - Edos getting along way too well. Ru telling the orphans stories her grandmother passed down, Yuel defending Monaty from Elka's bullying, Potato Angel and Lipre cooking together, Leonard learning there is no easy way back home, Paffel taking Akane's job. I smiled like a jackass during all of this.
>Two new battles. Once versus a possessed Luvaid and another against the demon leaders + Mergitos. You learn that SN1's demon king summoning awakened both Mergitos and Amer. Apparently the supposed demon king オールドレイク was attempting to call was just some low-level schmuck lol
>You get to use a huge portion of Summon Night 1's cast! You keep them even for free battles and the mugen kairou.

All in all an extremely satisfying alternate end. Some portions are kept as canon which is cool, too.
Still can't be assed to get through Mugen Kairou. This grind is awful even with fast forward.