Yeh, that's exactly the problem. (and one of the PITA moments you get with Mint)
You have 2 options ..
1) edit your sources to say
For both libreoffice PPA's (sources and binary)
2) as you say, download the .debs and install
If you decide to want to go with option 1 .. post the output from:
ls -a /etc/apt/sources.list.d
and the if it exists, the contents of: