Interesting read. On my first playthrough, I had to restart because I skipped too many monsters in the beginning and was too weak as a consequence. The second time, I did quests in the city and by the time Robin (?) joined, her level was too low compared to the rest. That‘s when I quit the game because it seemed like you ought to play exactly like the developer envisioned.
I don't remember this moment when she joined... but is the lower level even a problem?
This isn't the only time that someone joins your party while being underleveled.
But as far as I noticed, she could die just as easy as chars nearly 15 levels higher.
Aside from learning skills, levels don't mean THAT much.
If you think a stat is lacking for someone, balance it out with equipment.
If you get destroyed in a battle, change your strategy.
I'd say the dev envisioned that everyone plays how he/she wants, giving a fair bit of leeway here and there.
Some optional areas can be tricky, together with some bosses.
But I'm at a point where I couldn't care less if someone misses out on XP, cause I've beaten everything before recruiting said char.
And with how the XP distribution works, lower level char catch up fairly quickly.