I'm not sure I understand what domain authority means. I think of it in simpler terms. If inner pages are about a broad range of phrases relative to shorter phrases, and linked to because of those broad phrases, then it makes sense for the relative hub or home page to rank for the two word phrase.
That is one reason (but not the only reason) that it's important to bring links to the inner pages. In my opinion, the quantity and quality of links coming into the entire site might be what gives it authority and hence why it's more eligible/deserving/relevant to rank for the shorter phrases.
What is trust? I see that as the absence of certain negative signals. Those signals exist in the outgoing links and incoming links. In my opinion it's a quick way to determine the intent of a site and lump into a bucket associated with sites that are or are not useful. From there, imo, the site is free to move on to further scrutiny. From my experience of watching the SERPs, content is outranked by the link metrics.
The problem with pointing links to the home page is that all the sub-pages receive a share of that PR. But that kind of linking doesn't reflect the life of an average website. What is often happening is that some pages in a site are more important than others. In my opinion those pages deserve more links because other pages may be out of date, have been superseded by better information and many other reasons.
As far as ehow, aren't those links nofollowed? Might want to read the link dev library
[webmasterworld.com] and expand the kinds of links you are obtaining.