1. WinSCP makes things easier. But if you don't have wifi at home then we'll have to do it another way.
2. Yes
3. Getting the PSFreedom GUI application version to install on your N900 without the n900 itself being connected will be troublesome. PSFreedom-GUI depends on other packages also. So you'll have to pin point and download those. troublesome. specially on 3G.

4. Looking at your situation. Here is what I think you should do:

1,
Root your N900. (Give yourself super admin privileges)
http://maemo.org/downloads/product/Maemo5/rootsh/

When you click on the download link. You get an .install file. Transfer that to your N900. You can do that over USB. Just plug in your N900 to your Windows PC. It'll show up. Place the file anywhere there.

Unplug your N900 from your PC. (You have to do this, so don't get trouble in the next step)

On the N900, using the file manager, go to that place you put the file. Click on it. Application Manager will pick it up and try to install it. You need to be connected over 3G. It's a tiny package. so it should take a few seconds.

Close Application Manager and File Manager.

2.
Plug back your N900 to your PC. Now transfer the 3 PSFreedom Files from my first post (USB). (2 .sh and 1 .ko). to the N900.

3.
Launch the Terminal on your n900. Type:
Now we need to navigate to the place you put the 3 files. What usually shows up on windows when you plug the N900 is MyDocs....so.... in the Terminal on the n900 (Case Sensitive):
To know whats currently in that directory. Type:
4.
Finally once you confirm you're at the place where the 3 files are. Type: