Quite often it happens that when I'm on develop starting something that it gets larger than initially expected. Then creating a new feature works perfectly, so I reckon it seems to depend on what branch you actually are starting to work.

You already can do that. Just select both files and invoke Query | Show Changes. You will be asked whether to "Compare with Each Other" or "Compare with Repository".

How to do that for .zip, .tar.gz?

There is already the option to just clone the last x commits.

Where you want to get this option? In the Merge command?

Depending on the selected commit, the tag list already can be quite long. Showing the long tag annotation would it much harder to read. Instead it could make sense that the Details view could also work for the selected tag in the Branches view.

According to my knowledge Git does not allow to edit tags or a tag's annotation. To view the full annotation text (or copy it), please see this blog article as work-around.

You already can do that with 17.1 preview from http://www.syntevo.com/smartgit/preview and by invoking Help | Check for Latest Build. If the Changes view is focused/selected and you invoke the Index Editor, it will occur at the same place at the Changes view.

We've tried to implement this feature, but found it not easy and intuitive to use. Instead, the next 17.1 preview build will allow to open the Index editor at the same place where the Changes view is, so you can edit there.

I'm not sure whether I should select now Declined or Completed - I use the latter. ;)

There is nothing "attached" to commits in Git. The detection which Git ref has this commit in its history takes some time and it would not help in usual back- and forth-merged branches.