I looked into it, adding custom chars seems possible. But it needs all the assets, coding of the events and more. Nothing I am capable of at the moment. (Especially the asset part)
Textures and events are indeed a hassle. However, based on my experience modifying memory in other games, some characters, even without access to player mode, seem to be able to be summoned and used directly.
Here's a small idea: the logic for the guild adding player characters to the party should logically be similar to the logic for recruiting mercenaries at the guesthouse.
Can you try copying the target location event sequence for the guild's join player party event to the guesthouse's mercenary recruitment event? It's only the final address and number that will differ, so it should be possible to forcibly summon them to the main party this way.
If possible, try checking whether their backpacks, bracelets, and rings function properly (weapons and armor with paper doll textures might have issues), and also test whether the mercenary characters can be used normally in battle.
If these basics work, the summoning logic for summoned beasts should also be able to forcibly summon them to the main party using this method. If everything is fine, judging by the demo version forcing a call to the heroine, the issue is likely related to the tavern error event. I know the previous mod author fixed this, but I have no idea how to do it.
Unfortunately, I'm not a coder, so I can only infer the logic behind address modification in CE.
I wonder if you'd be interested in trying it out? It should only require modifying a few event addresses to achieve a rough result.