Your options pretty much consist of 1 and 2. The others are either potentially dangerous (3) or needlessly expensive/complex.
Using a dedicated 3rd computer setup to host the files with filemaker pro for you and your secretary to access via Open Remote... is the simplest to set up.
Using IWP is cheaper, but IWP has built in interface limitations due to requiring a web browser for accessing a file and a bit more effort getting everything configured correctly. It would offer the possibility that you could access the database from any computer while traveiling, however.
I hesitate to suggest this, but there's a third possibility:
Host the file from your computer while you are in the office, but drag a copy onto your secretary's machine when you leave and drag her copy back onto your's when you return. That works, but you end up with different copies of the data on two different machines with the very real chance you could both end up accidently working with different copies of the same file...