I just bought a new nexus 7, and I have a galaxy s3 phone. I had the same problem of not being able to transfer easily with my mac with the s3.
Happily, the nexus 7 transfers great with the mac once you download and install the free Adroid File Transfer software for your mac. After the first launch, it will automatically detect your (nexus?) device and then you can transfer normally.
I'm also using Calibre and Calibre companion. I just bought the Moon+ pro version (on sale this month, half price!) and have not had the problems you're mentioning.
I would recommend that you try something different within calibre -- if you have it all displaying the way you like in calibre, you can try (pick one

convert to epub, modify epub, or polish epub - and be sure to embed the metadata information into the epub file. You can choose to either embed or remove embedded covers from the epub file as well.
So - then when you go to Calibre Companion, the metadata information available in the software (when you choose to load it to your reader) will also be available in the book itself when you open it in Moon+ on your ereader.
I've decided this is the better method than, in calibre, saving metadata and covers in separate files. I spent so much time fixing and re-fixing ebooks that way, because it seemed calibre either kept forgetting, or kept ignoring, metadata information stored in the same directory as the ebook itself. (was it clugy? or was i? i don't care... it sucked.)
The first time you open the book, whatever the ereading app, you should then see the cover (if you embedded it) and a metadata book jacket with description and any series, index you may have in the database. subsequent launches of the book will be at whatever reading point you stopped last, you shouldn't have to see the cover and book jacket information more times than you like.
hope that helps.
---
ps try android file transfer for your galaxy tab. it may help. if it doesn't, try loading Kies for Mac on your computer and the Kies app on your galaxy. I didn't like the interface AT ALL but, it does work. there's also Kies Air for wireless transfer, but that was arguably even worse, in my book.
If you CAN trade out the samsung for the nexus, it might be worth it if you do a lot of transferring with your mac.