1. Yes, your characters should be aware of each other by now. This is related to 3, as you've all been Substitute Shinigami for anywhere between 1-2 months already. Maybe 3 months even.

2. You may have heard of Shikai in passing, and what it is, but the recruiters had never had any need for them to unleash theirs as demonstration. Though their first showing will happen in the first arc.

3. A few months give or take as said.

4. An artificial soul is provided by a Shinigami scientist that runs a candy shop in Sagamiyama. They can have very different personalities from your character for fun, or you can play them seriously if you like. Up to you. It's done by eating a candy which inserts the soul into your body. I will note, you do have a Shinigami seal also, but using that will not allow you to put in an artificial soul, someone else would have to do it for you.

As far as that Shinigami scientist is, I have yet to finalize them so you'll only meet them later.