She's dying, and needs them to come to the lake, but I am too weak to fight the douches right besides the lake. Will it mess things up if I leave and come back?

No. The White Witch isn't even at the lake. Take your time.

Thanks, but it worked out, didn't realize I had soo many nice scrolls to use :D

Nope, but it kind of feels like it huh?

"The woman of the lake" keeps telling you to hurry! Before it's too late!

Only quest I know that fails if you take too long is with the Frederick guy that fell off the cliff. Are there any others? Haven't come across any.

In Cyseal, the burning ship will indeed burn up and sink if you don't put it out quickly enough.

The decision at the healer's home - which of the two townspeople should Evelyn save - can also end with both dying if you dally too long before deciding (which you can do by exiting the dialogue and wandering off).

Wow really? Evelyn is so incompetent that she can't make up her own mind, and need to rely on the word of a random stranger who just happened to walk in the door? And if the random stranger leaves she just let them both die? O_o
It would be much better if she chose one randomly if you happened to walk away. IDK how her blood/star stone could be solved this way, but it would make a lot more sense imo =/

It's actually rather reasonable for her to not make up her mind about it considering later developments.

Hmm werent they using the stones to prove the healing abilities and gather more people following them? =/ (or am I missing something? ^.^)

Or if nothing else: it totally gives her away =/