I know exactly what you mean. My dreams are so detailed and long that I believe I'm actually living it, and it causes me problems when I try to wake up. It's like my dreams become my true life and going back into waking life is really hard, sometimes it's as if my waking life is the dream. I've spent what felt like many lifetimes inside dreams.
As for it actually being an alternate universe, I sometimes still think that way and want to believe it, but to be honest if I'm not careful it fuels delusions of mine.

Also occaisionally I've woken up thinking my dream was still going on, or thinking that I was actually awake and really experiencing those things instead of just dreaming them. The first and scariest time that happened, I woke up with ants covering my body and in my bed and room. Other times are too gruesome and creepy to say here.

I have deja vu about 90% of the time. What you said about predicting conversation has happened to me too. It's unnerving. I know that it's a complex thing and you can get many different answers from all types of people. What might help is to try keeping an open mind and a positive/ hopeful outlook- this helps me when the feeling of deja vu overwhelms me abd I become preoccupied.
It might be possible that our brain has a certain chemical reaction in certain situations, for example when you're sitting with your family your brain might be chemically reacting and saving the specific chemicals to your memory, so that when you do the same activity it releases the same chemicals again, thus creating deja vu. (this is just my speculation though)

I've also experienced seeing things that no one else has, and save for a few experiences I think the rest are hallucinations. Could it be possible that you were hallucinating the spider and ants? Perhaps you were more stressed than usual when it happened?

Have you experienced abuse or anything traumatic, or otherwise very afflicting? Do you have someone- a doctor or therapist or anyone you trust to be non-judgemental and supportive - who you can talk to about this? It may be possible that you have a sleep disorder, and in fact this is the same thing I need to bring up with my therapist.
If you want to look into the symbolic meaning, you can use dreammoods.com.
Another way would be to think of what a spider/ the symbol means to YOU, instead of using an outside defined meaning. Then think of YOUR created meaning for the symbol and see how it relates to your dream and your waking life.
Do you keep a dream journal?
You can also message me any time you want to talk, if you need to. I hope you're doing alright.

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