There are 2 options for you.
Your mother can purchase the HD's and register them to her B&N account, then send the devices to you. She can purchase the ebooks for you, and you can then download them wherever you are that Wi-fi is available,
We have a daughter that lives in Ireland, and uses the nook there. Here is what we had to do to enable her to buy her own books from B&N in the USA. We registered the nook to her by purchasing a refillable credit card as the credit card number on file, and using her google email as the email address, but using our home address as the physical address of the credit card. She sends us the money to purchase egift cards for her account so the credit card on file is not depleted. If the ebooks from B&N are free, she can obtain them directly on her tablet in Ireland. If they are not, she emails us what she wants purchased, we log into her account on our computer and purchase them for her, then she downloads them to her tablet.
I can't help you with the Google apps, as our daughter does not have an HD...
For question #3, I don't believe you can purchase them on your desktop computer in Europe.
Yokel, I have sent you a Private Message (the small white envelope on top of the page).