This belongs in the InDesign forums but I can sum up most of the answers you would receive there.
You need to work with a competant provider for the magazine. Period. That is not going to happen.
Yes, a pdf file is perfectly acceptable for use in an InDesign document. It would be the norm if you were supplying ad copy - but as I understand, you are writing article copy. If they have style sheets in use, there would be a potential conflict of nearly everything; column width, font styles, margins, run in, etc. and you page(s) would not match the balance of the magazine.
If they were savvy, you would use InCopy. InCopy allows writing and editting of InDesign text content where many individuals are required to get the content written; the final layout and graphics are handled by one user (obstesibly), one final InDesign file.
Your use of unique styles would work if they retained formatting upon place; they may choose not to, or they do not know the Paste command modified to retain formatting. But, you have seen the danger of using soft returns as well.
To your questions.
1. PDF is perfectly acceptable for Place and use in InDesign. (And since CS3 you are also able to place a live InDesign document as well) The magazine may prefer to format and allow reflow however. PDF locks that ability.
2. InDesign documents see no text engine or document changes relevant to OS within the same version. Font availability/matching is the caveat. Use of OpenType fonts allow the same font to be used in both OS's. I may be less than correct here, but - Mac OS is able to use PC True Type and Type 1 fonts if they are placed in the InDesign > Program > Fonts folder. The equivilant is not possible with Mac fonts on a PC due to Mac's research fork type file format.
I hope this all helps. The InDesign forum is full of helpful individuals who can elaborate on all the above.