Sounds like you rank for a lot more terms on desktop than mobile, and probably not as well for a lot of those terms. In addition, it could be that a lot of the mobile searchers are in your area, which tends to make you rank higher because of the localization.

With questions like this, I tend to look at it not from Google's point of view but from a person's point of view. The spiders are getting smarter after all and Google always says to write content and create websites for people not the Spiders.

So to answer your questions, you might want to ask yourself these questions:

• How am I supposed to know that the links on my website are broken, because a site I was linking to is now down?
• How do I know that the domain I am now visiting was down for a month or even a year?
• How do I know said blog is being used for black hat purposes? It it has relevant content and helps me, that's all that matters.

I would say use hyphens in the urls.  I've never known there to be a ranking difference and in my opinion it's a best practice to use hyphens over underscores or plus symbols, and for readability for the user I think hyphens make sense. texas-plumbers reads easier than texasplumbers to a user, and for Google I would think it's easier to discern.  Imagine some titles where words could make completely different sentences by separating them at different locations.  http://www.boredpanda.com/worst-domain-names/ as some examples.

Yes, you will want to redirect it. Be careful though as a lot of times this isn't done correctly and creates a loop. There are multiple ways to do this including but not limited to DirectoryIndex or a RewriteRule