Your story is indeed interesting but it could also be the result of the power of suggestion.  If you believed that you would suffer consequences from killing the spider, then you set yourself up for it.  Then, if you believed the Budha could rid you of these dreams, then you allowed yourself to not experience them anymore. That may not be the truth, but I tend to go with the simplest answer first. 

Do the marriage problems have to do with your beliefs? 

Why do you feel you have a spirit watching you?  What have you experienced?

My feeling is that you don't know if she is truely there or not unless you just act like and believe she is.  Then experiment.  You can try a normal physical method of using a tape recorder on and asking "is anyone there?", or leave it on all night and see if you get anything. 

Or you can try to utilize some of the non-physical means of communication like using your imagination to bring the spirit to you and communicate that way.  With this process, you have to trust that whatever you receive in your imagination is real and just play along with it.  With this method you try to find out information about who she is and what she wants.  Maybe you would find out she is looking for someone, a lover or friend, a child, etc.  In your imagination you can tell her you know this person, for example, and that you would like to bring this person to her right now.  In this way, a "helper" would show up (a non-physical being from the afterlife) and she can then go off with that person, thus "crossing over". 

You can also try the method of asking guidance for help in knowing what to do, you could meditate and mentally send her away, or you could really make up any of your own methods which I'm sure would work based on intention alone.  It doesn't hurt to try. 

I recommend Bruce's method because I support it and also because you can learn so much from it.  It will open your belief and perception of the afterlife and non-physical reality.