In regards to question #2, make sure you have Exact Match enabled.  The default setting is Broad Match, which groups all related keyword together.

In the left sidebar, look for the drop down that says "Match Types".  Uncheck the box marked "Broad", and check the box marked "[Exact]".  This will give you the search volume for the exact keywords that you typed in.

For example, using your examples, "photo booth props" has an exact global search volume of 5,400 while "props for a photo booth" only has 36 searches.

Like others have said, you have to wade through a lot of noise to find the nuggets of good content on the Warrior Forum.  However, the paid forum has a better S/N ratio than the free forum, and it's a fairly inexpensive one-time fee to join, so I think it's a pretty good deal.  I am personally a member.

Well, Google isn't going to punish you for owning 4 different websites, it's perfectly fine to own multiple web properties that drive traffic to your business.  In fact, you're diversifying your risk by having multiple sites, since if one drops in rankings, you still have 3 others.

If the other sites are spammy, why would you want that content on your main site anyway?  Just include links from the 3 other sites, and point them all to your flagship site. That way your main site still gets the SEO boost, and you can build it out however you want, while you don't lose traffic from the other 3 sites.

Sounds like a big and risky endeavor, although there could be potential benefits.

Why are you considering merging the sites if they are already ranking well for their keywords?  Are they not ranking high enough?  If it ain't broke...

A better strategy might be to just build more links and add more content to the sites you have.  Another area you could focus your efforts on is conversion rate optimization.  You're already getting all this traffic for desirable keywords, so you can boost your client's ROI by figuring out how to turn more of that traffic into leads.

That's what the css code above does, it puts the link beneath the image visually when users look at the site, while keeping the link above the image in the actual code.