#1 I have added this missing phrase to the next release.
- vw_index_permissions_canlist_title: Can view list of content located on the wiki index?
#2 There are no "widget." phrases in the XML. These phrases are generated automatically by XenForo whenever you create a widget on your forum. They can be translated but you should not associate them with the VaultWiki add-on. Some of them are for XenForo and some are for other add-ons. If you do associate them with VaultWiki, they will automatically be deleted on your next upgrade, because they are not included in the XML. And then you will have a bunch of widgets with no text. VaultWiki cannot guess what widgets you will always have and what you named them, even if the widget-definition is provided by VaultWiki, so we cannot put them in the XML. The same is true for other phrases automatically generated by XenForo like BB-Codes. Since VaultWiki lets you rename your BB-Codes, the phrase names can't be known.
#3 It is not entirely clear, but I think you mean, the list of Special pages at Special:SpecialPages. This list shows the titles of the Special page content. These are not phrases because they are content. In order to translate the title, you would have to edit the Special page and change the title. Theoretically we could make a separate language file for pre-installed content, that reads a translation before it first gets installed, but it seems like vanilla XenForo doesn't even do this for its own pre-installed content.
As for the language file being big. Yes, VaultWiki is a big product with many features, approaching the code size of even a forum by itself (XenForo alone has over 10000 phrases). I agree that there are probably some unused phrases at this point, and now can be a perfect time to investigate that. But it will not be a lot. And next major version you can probably expect 1000 or more new phrases. So the size will only continue to grow as we continue to add features.