The seal is basically an all-or-nothing function. If part of it is broken, the other parts try to use the now-unused power to power the fuck up and keep shit sealed anyway. This works somewhat- It is an incredibly temporary measure, and is basically there to keep shit restrained while someone who can comes and repairs the broken seal. On the other hand, if this happens and no one comes to fix it, it's basically got a gaping hole in it and throwing all of its power into that hole, so it's just going to wear and erode itself away faster.

If the seal came off, no, Tatsuki really wouldn't know any of it. From her point of view, it would basically feel like she was asleep and having a really hazy bad dream. She wouldn't be aware of anything past a vague feeling of unrest and unhappiness, but be too fogged and hazy to understand any of it.

Other than the Saiga and Yasu thing, is Erland looking to add anyone else to his harem? Who all is in it?