the limiting factor in your scenario is the upload bandwidth of your mother-in-law's internet connection at home. there is no way around this limitation, aside from upgrading her connection, but generally consumer/home internet connections have a very limited upload speed.

directly editing documents to a pogoplug over the internet is not recommended. if your connection hiccups in the middle of a save or something like that, you may end up with a corrupted document. the safest way to edit your files remotely is to first download them to your desktop, edit them, then upload them back to your pogoplug.

sounds like your setup should work. if you have 2 routers chained together (meaning, a router plugged into another router) you may have difficulty accessing the pogoplug from the interior network. i can't tell from your description if this is the case.

you want to have the pogoplug attached to the same router to which your pcs are attached. that is, if your pcs are connected to the wireless router, then keep the pogoplug attached to the wireless router. but you may have difficulty accessing the pogoplug from pcs attached to the "outer" wired router.

basically if the pogoplug desktop application is working and you are getting good speed to the pogoplug from inside your home network, then you have it set up correctly :)