First thing to do is to head to your college's library and ask the librarian whether your college offers Microsoft Office for free or a token charge. Most colleges make this offer to their students.
If your college doesn't participate in that program, there's a different program from Microsoft that gives Microsoft Office to college students for a nominal fee
http://www.microsoft.com/mac/office-university
Now on to your questions:
LibreOffice is the most up to date fork of OpenOffice. Use LibreOffice for the best compatibility with Microsoft Office. Just the same, there are literally hundreds of features in Microsoft Office that LibreOffice doesn't have yet.
LibreOffice is the same on Mac, PC and LINUX.
The history of OpenOffice/LibreOffice explains the "something up" uneasyness you feel. Before OpenOffice was free open source software it was a commercial product sold by Sun Microsystems. The product bombed out in the market place, and Sun continued to pour mega $$ into development, but to no avail. To spite Microsoft Sun decided to give it away for free. After sinking $$ into OpenOffice Sun went belly-up. Oracle Corporation bought Sun's OpenOffice assets and immediately decided to jettison OpenOffice. A splinter group of the OpenOffice team made LibreOffice from OpenOffice and is trying to keep it alive. Without a sugar daddy, this will be difficult to sustain, but they are valiant and the effort continues.
In theory, it is possible to get a macro virus from LibreOffice. As a practical matter, the macro tools in LibreOffice are pretty bad, so getting a macro virus to actually run would be an accomplishment all in its own. So while it's theoretically possible, it's extremely unlikely you would get a virus from LibreOffice.
What you do have to beware of is the install process for LibreOffice and OpenOffice. You'll easily blow by the little box checked by default that will install something you don't want or need like Google Chrome, a web browser toolbar, or other goodie that LibreOffice gets money for pushing. Look for a check box and uncheck it in the web page or installer so you don't get other junk along with your LibreOffice install. They get paid to stick you with unwanted, unasked for stuff.
Yes, there is some harm that can come to your computer. If you already have Microsoft Office installed, LibreOffice and OpenOffice will change all your file associations to their software even if you don't ask for it. In fact, every time you open LibreOffice or OpenOffice it might change your file associations even if you changed them yourself to Microsoft Office.
Yes, you can view old saved Word, PowerPoint and Excel files with LibreOffice. Where you might run into trouble is viewing current Word, PowerPoint and Excel files because the LibeOffice OpenOffice feature set hasn't change a whole lot since the days of Sun back in 1995-1997 timeframe.
If your major involves data analysis, you can skip OpenOffice/LibreOffice altogether. The new features in Excel are simply not in the free products.