Well, that seems more like a business strategy question than a technical one. You need to decide if you are still going to operate the companies separately or are going to merge one into the other. If you are going to operate under just one name and domain I would make a big marketing push to let everyone know that you are merging the companies into one and that this is our new business strategy and "Domain Name".

Then after some time has passed I would start redirecting all the content and traffic to the "NEW" site. with the standard 301 procedures. Making sure that everything you are keeping is redirected properly will help keep about 85% of the "Link Juice". There is always a little lost basically from how the algorithms work, but that is out of your control.

Hello, in WordPress because everything is based on the Database, in my opinion, it is not helpful to have an extraordinary amount of unpublished pages. If the pages are unpublished and are not indexable you should be ok with the search engines, but I think it is not a good practice to get into. Also if the unfinished pages are resource heavy and have a lot of images that take up space it may start to slow the general operation of the site, especially if you are on shared hosting with limited resources.

I feel that the search volume shown in MOZ is generally low compared to other sources. I feel it is more accurate with search terms that have a lot of volume, but in search terms that are under 1000 searches a month, I start to have a lot of problems with the data. Many times there is no search volume data at all for lower volume keywords which get just a few hundred searches per month or less.

Problems with the volume data effect almost everything else in Moz so I hope MOZ sees this and tries very hard to correct these issues.