I would go further than that and check the link profiles of all the domains.  Any sign of spam, unnatural anchor text etc then do not redirect as you'll inject that problem into your site.  Even if you believe them to be dormant and never used I would just check in case you were not the original owner.  Always worth doing that check.

ScreamingFrog gives all the data you want.  Tools for the purpose of a creating a sleek report usually don't give the full picture.  It's those issues you draw out yourself that makes a difference.

I second ScreamingFrog.  Nothing else comes close.

You could legitimately have hidden content in terms of category/URLs - maybe only made available if you sign up as an example.  The guidelines are referring to manipulating on page content so there are elements within that page that are hidden and by default manipulating.

Andy is spot on.

Stripping it back to the bone the basic rule is that you violate Google terms by purchasing links.  It is not an instant Google hit if you do, but you kind of move into death row and take your chance.

Because you don't always instantly suffer a penalty doesn't mean that you should follow any advice that seems like a quick fix.  I would stick to here or the other main marketing sites to get a concrete base of what is ethical, what works and what doesn't.  That's not to suggest you shouldn't implement your own strategies but they should be built on that concrete base.

I think it really depends on your blog, the market and what you want to achieve.  A tight niche could be very lucrative if you become the authority.

Say a keyword has approx 100 searches per month and you rank well and achieve a sale/signup/commitment per month from that.  Then there are 30 other search terms with similar volumes you can rank for.  Is this valuable to you?

If you are selling 'blue widgets' at £1 per widget and a typical customer only ever buys one or two then your niche blog may not be worth your efforts.  But if they are repeat purchasers then again that changes the landscape.

Any URL that you want to remove from a particular TLD needs to be done in the specific TLD Webmaster Tools property.

If the page ia a 404 not found it will sometime soon disappear from the index.  You can force this immediately by going into Webmaster Tools >> Google Index >> Remove URLs >> enter URL >> select 'Remove page....'

The first thing I would do is look at the main menu as is is broken giving you lots of 404's.

training-courses/ and all the ?active variations
faq/
flagger_certification/
osha-violations/violation/

I have run the site through ScreamingFrog.  You have masses and masses of 301 redirects going on which you need to resolve.  It looks like you could have some permalinks issue going on.  What do have set in Settings >> Permalinks?  I'd also look at installing Yoast Wordpress SEO as I recall there are options in there to handle permalinks as well.