I think Tuvok is the perfect person to talk to and take advice from when your emotions are a mess and you need a rock to hold onto. He is that rock..and I think thats what Janeway sees and appreciates. And like you guys said, since he is able to look at the situation objectively he can offer some other perspectives that Janeway couldn't/wouldn't see.
Btw, Tuvix is one of my favourite episodes...I think its astonishing that they wrote a character and found an actor to portray someone so likable in such an incredibly short time. Despite myself I found myself rooting for Tuvix in the end. I really was sad to see him go. He was such a unique character it would have been interesting to see him develop and adapt further.