With regard to #6, and also to ken_b's question, it's perhaps helpful to "triangulate" searches... ie, try to get different angles on the same subject. Compare, eg, the following terms, including the originally suggested "economic depression", to each other or in various groupings, or individually, and consider what the trends might mean....
Here are some sample comparisons....
unemployment,economic depression [google.com]
If you compare "unemployment" with "economic depression", you'll see that "economic depression" practically flatlines, whereas "unemployment" searches are clearly much more frequent and are on the rise.
unemployment,employment,economic depression [google.com]
If you add "employment" to these two, you'll see that search volume for "employment" was initially higher than for either of the above, but is trending down... whereas "unemployment" searches appear to be trending up. It's likely that volume on "employment" and "unemployment" searches will eventually cross.
economic depression [google.com]
economic depression,recession [google.com]
economic depression,economic recession [google.com]
economic depression,recession,unemployment,employment [google.com]
Comparing "economic depression" with "recession"... both have roughly the same pattern, but with "recession" peaks preceding "economic depression" peaks. Both peaked several times in late 2008 and early 2009, and then both trended down. As would be expected, the single word "recession" is much more frequently searched than the two-word "economic depression"... so direct comparisons on the chart are hard to make. Note that "economic recession" and "economic depression" match each other fairly closely. "Recession" is still much less frequently searched than "employment" and "unemployment".
Throwing in a four-letter word, "jobs"...
If you compare "employment", "unemployment", and "jobs", you'll see that the search volume for "jobs" is much higher than for either of the employment terms, and is trending up. As noted above, the searches for "unemployment" appear to be overtaking those for "employment".
unemployment,employment,jobs,economic depression [google.com]
If you add "economic depression" to this 3-way comparison, its relative volume is so low it doesn't even appear on the chart. Changing "economic depression" to just plain "recession" (just plain "depression" would be ambiguous without "economic") also has "recession" way under "jobs" and noticeably under "employment".