If I want to search for some tag, and I have not any so tagged post in front of me, how I open posts with that tag?

I know that if I have one similarly tagged I can only click the tag.

Maybe one text before the tag cloud, or higher level in hierarchy with possibility to get result only in that subtree instead of everywhere or same as option of site search could search only tags.

I think you're the second person to ask for a search on multiple tags. There's a thread somewhere about it.