I am inclined to lead toward some type of siloing with a high content site. There is the very purest silo architecture which I feel Bruce Clay presents very clearly in his site articles. You can certainly vary it to be less rigid and still be an effective SEO tool.

I generally agree that MOST content should not be too many clicks from the home page, but drop down menus can go a long way to keep a lot of content close without it being unwieldy. Perhaps it will help to look at it this way: the way your structure your navigation tells Google what you believe your most important pages are - if you tell them ALL your pages are equally important, you dilute the ability of your top pages to rank better than your lesser pages.

If that makes sense to you, I hope it helps.

301 redirects are the very best way to redirect and retain the most link power. Within the site, you have nothing to worry about if your new structure has better SEO. 301 redirects do not always pass 100% of external bank link juice, but it's still the best tool we have to keep what we have already achieved.

It shouldn't; search engines can isolate strings without separators. However, how many people do you think actually type 'girls pink yoga capri' or are you just being hypothetical?

No reason too worry, as others have said. I  don't advise nollow, actually - in general, I believe comments should be moderated; if they are spammy they'll never be followed because they won't be approved. But a clearly stated moderation policy that lets readers know that substantive comments are appreciate - and rewarded with a followed link - encourages good contribution, IMO.

Matt Cutts recommends against using nofollow this way -- now, you might want to take anything Matt says with a grain of salt, but we do know that you cannot control the flow of PageRank in this way. Your other pages will not get more PR because you have nofollowed others; instead Matt says, the PR is lost. See the video

That said, a larger site might benefit indirectly from a judicious use of the robots.txt file to prevent the indexing pf pages that are not likely to end up in the SERPs and allowing more of the site's crawl time allotment for pages that have SERP value.

watch?v=4SAPUx4Beh8

I have never had success in getting a link from or even an idea picked up by/through HARO. And I do think that I have offered some useful tidbits, etc, but there are a lot of others offering useful info, too.  The service is probably more effective for journalists than for SEOs or webmasters. I  am US based.