Some things I'd need advice with

Now, as you may alreayd know, I've had an idea for a story for a long time, and I'm finally starting to write it. I have most of the plot worked out, but there are some small details that I've previously skipped because they weren't that important to the outcome. Now that I've actually started writing them, I'll have to do something about them or I end up with plotholes, tho. The story is Urban Fantasy type of thing, by the way.

First thing is that the main characters have a mentor figure of sorts, a man who has a lot of knowlege of various supernatural things that most people are unaware of. However, I'm not quite sure how to get him to become their mentor. The basic idea is that he's a professor at the university, and one of the characters goes to ask for some information, since he knows the professor is an expert on obscure mythological figures and such. However, the problem is that at that point neighter knows that the other person is also aware that supernatural beings exist.

Secondly, in another story, one of the characters gets captured by cultists that want to use her as a sacrifice. The other characters will obviously come to rescue her, but the problem is, I'm not sure how they would find out that she's been captured in time. I figured I could subvert the cliche of cellphones always being useless and just have her call for help, but that raises some other issues, namely that to make a call she would have to be unrestrained, and since she would be quite cabable of kicking the cultist's asses herself in a fair fight (the only reason they caught her is because they took her by suprise and knocked her out before she could fight back), it would make sense for them to keep her tied up.

Also, there's some minor things I'm wondering about, namely a) how would one force themselves to wake up from a dream (assuming they are aware that they are dreaming)? and b) can those flare guns used to send emergency signals set something on fire (more specifically, if you shot a person with one, would their clothes catch on fire)?

Dream-wise, causing "pain" or falling can usually end a dream. A shock, basically.

When I want to wake up I shut my eyes as tightly as I can and then open them. Always works.

As for the mentor, the characters may go back to him several times. Eventually he would offer, or they would ask for, guidance on a formal level. It's not like he has would-be disciples knocking down his door.

It should be possible to develop an app (if it hasn't been already) with a dead-man switch. You turn it on and keep your finger on a button. If you take your finger off that button and don't deactivate the app within a certain interval, it calls someone and gives your location. It's fast enough that the bad guys can't stop it (even if they knew about it), but vague because there's no message, no explanation.

Flares can have different ingredients depending on application. If you were shot with a flare gun, you'd have the normal powder burns that you get from a hand gun (they both use gunpowder as the propellant). You'd also have a slug of chemicals inside your body that is designed to burn at high temperature, so burning clothes would not be a priority. I'd say clothes would probably not burn.

Sadly, those really wouldn't work.

While the professor does't really have to become their mentor in the first appearance, they need to get his help in that chapter. Basically, one of the characters is aproached in his dreams by an entity (which is actually the Herald, the BBEG of the story), and the other characters end up asking the professor for some information since they don't know what it is. I'd need to find some reasonable way for him to realize that they've actually encountered the being, after which he offers to help them. I could always just have him immediately figure something is off and ask them about it, but that might seem a bit too much like jumping to conclusions.

As for the phone thing, a "dead man's switch" app wouldn't really solve the problem. I was thinking of just having the cultist throw the character in a cell, which allows her to make the call before they notice and break the phone, but the problem with that logically they should just tie her up after capturing her, instead of putting her in a cell unrestrained (because while she'd be unable to escape the cell, it'd be quite difficult for them to get her out of there to be sacrificed). I suppose that since this is a fantasy story and all, they could just use magic to control people, but it seems pretty inefficient to do that when simple chains would be sufficient.