If it all goes to plan and both systems can co-exist, will the Win 8 be able see files in the Linux partition and vice versa?
Not easily - Although you should be able to see all the files in the Windows partition from Linux, Windows doesn't know about Linux file systems (I believe it's ext4 by default on Mint). In Windows, you can use a variety of things to browse the Linux partition (google for "ext4 windows").
I guess what I really want to know is, if the machine ingests any trojans/virus/malware in one OS will the other be protected?
Yes, this is indeed possible - Windows programs and Linux programs won't run under the other one. Actually, that's a bit of an oversimplification, but it's not something you should worry about, especially as Linux doesn't have viruses in the same way.
Should one boot fail, will the other be able to recover any data files/picures/videos/documents etc?
Yes, this should be more possible, see above for methods of accessing the data!