Hmmm good question 🤔

Honestly the first thing that comes to mind is that Seele's lock represents her not being able to use the full power of her stigmata. You can also interpret it as her being 'trapped' in the sea of Quanta (or in their shared room) .w.)

I still think that the Key of Acheron is the key to her lock and nobody can change my mind :^D

As for 'Seele' and red eyes hmmmmm, maybe she has that cus she's always watching Seele from the television in their shared mindroom w. Or maybe it's cus it's her first time to fully materialize outside of their shared room and see the world? who knows -w-)

Also oh wow this question's a year old, sorry for the (very) late answer < <);;;;