Let me start off by sharing the description of the "Minimal Subfolders in the URL" metric from the On Page report:
The quantity of subfolders in a URL appears to correlate to rankings. URLs with fewer trailing slashes perform better than those with more. Additionally, search engine representatives have recommended that excessive, subfolders in a URL string may be a signal that the page is very deep in a site's structure and may be less valuable/worthwhile to crawl, index and rank.
Your On Page report, like many SEO tools, seeks to call your attention to possible issues on your site. There is a correlation to more folders being an increased number of clicks away from the home page. If that is a concern on your site, then the warning is valid. If that is not a concern, you can disregard the warning.
There are other reasons to shorten your URL structure other then rankings. Click Through Rate can be affected by URL appearance. It has been clearly shown users wish to know the URL of the page prior to clicking on the link. Your current URL cannot be seen completely in SERPs. I performed a search in Google.com for the URL path you shared, "/trails/Canada/British-Columbia/Greater-Vancouver-Regional-District/Baden--Powell-Trail/trail/2". Notice the result URL? It is cut off and you never get to see what is presumably the most important part.
Of your ideas, I like your preference the best as a URL structure, /trail/ca-bc-vancouver/Baden-Powell-Trail.
Whatever your decision, there is absolutely no need to stuff every keyword in the URL. mysite.com/baden-powell-trail works just fine. If the extra folders make sense for your users and site, by all means use them. I would advise against adding any keywords to a URL in a pure attempt to influence rankings.