B: If I change a symbol in Illustrator it should be updated in my Photoshop file
Check out this Panel:
But that is for placed Smart Objects, and would affect changed Symbols in ai-files naturally only if there were instances of them in the document.
Basically I find what you are asking pretty unrealistic* and would be surprised if an easy solution would be forthcoming.
Scripting might offer possible work-arounds that use BridgeTalk to get Illustrator content over to Photoshop, but that would probably be slow and not meet what you are expecting.
The possibly easiest approach might be to have a Script create a new Layer and place one Symbol on it for each one of the Symbols in Illustrator, then save a psd with Layers.
Why do you need the Symbols in Photoshop anyway, if I may ask?
Edit: *) unrealistic in terms of the functionality currently being realizable with a reasonable expenditure of resources; but I’m admittedly a layman with regard to that.
If it's just for client sessions, you really should look into doing this with InDesign/ Flash Catalyst/ Captivate/ Powerpoint or HTML based with Muse/ Edge to create functioning UI prototypes. You can always do the finetuning later and construct the final UI in PS...