Can I download newsstand in-app purchases via my computer?
I don't think that it is possible to download the magazine onto your computer. I have little experience with newsstand. 

However, it may be possible to take your (presumably ethernet) internet connection, and share it to a wifi card in a mac (if you or someone else you know who will let you tweak setting on their computer have one) 

On a mac, go to system preferences, and click on the sharing pane.
Click on the internet sharing option in the sidebar. From the drop down menu, select share connection from ethernet, then check the checkbox for wifi. 

This should theoretically enable you to download the magazine directly to your iPad, via the computers wifi.