IMHO You're overthinking it.
1. Get card reader
2. Get an external bluray drive like this one:
http://www.newegg.com/Product/Product.aspx?Item=N82E16827151272 or this one :
http://www.newegg.com/Product/Product.aspx?Item=105-000D-00001 (this one needs 2 usb 2.0 ports or one usb 3.0 port, probably for more power)
3. Find a laptop/computer (whatever) with two usb ports.
4. Copy contents of SD card to hdd.
5. Burn 1 or 2 bluray discs with the content of the card.
6. Put the bluray discs in a jewel case (plastic) and then in a cardboard mailer.
7. Mail them to your grandma/ to a friend at home / to your workplace (after telling them to expect mail from you)
The bluray writer is less sensitive to moving around (compared to a classic hdd) and is lighter and you can easily protect it from damage. You can get bluray disc in Japan from shops or you can buy a 25 pack from home and carry it with you, they're not expensive at 1-3$ for 25GB. There are some compatibility issues so you may want to buy some discs and make some tests to see if the media is compatible in advance. Here's an example of 25 pcs pack :
http://www.newegg.com/Product/Product.aspx?Item=N82E16817279007&cm_re=bluray_disc_blank-_-17-279-007-_-ProductMost of Japan has good internet connections (10mbps or more) BUT it's mostly fast speed within Japan.
You could for example rent a dedicated server with 1 TB of disk space or more for.. maybe 60$ a month and cancel it after the first month. For example see Hetzner in Germany :
http://www.hetzner.de/hosting/produkte_rootserver/ex40 - 45 euro a month, for 2 x 2 TB drives and it would probably cost about 10-15 euro extra for windows (so you could connect to it using remote desktop connection and setup a ftp server easily if you're not good with linux). They also have older hardware, for example some dual core AMD servers with 2 x 400 GB hdds for 25 euro a month.
Ideally, you could upload from an internet cafe to your dedicated server using ftp or some other method and then mail a friend of yours at home to download the stuff for backup on his computer until you come home.
The problem is there's a high change speeds outside Japan will be slow - even though your server in Germany may be connected to the internet with 1gbps link, you may only upload from Japan at 100-300KB/s, so a 64GB card would take ages to transfer.
A trick you could do would be to upload the videos (only videos unfortunately) to Youtube. If your camera splits the video automatically in 2 GB or less segments, you could upload the videos in a Youtube account (mark them as Private or Unlisted). Youtube probably has dedicated servers in Japan so uploads will be much faster to Youtube compared to uploading to a cloud or your own dedicated server.
You may think that you can't download what you uploaded to youtube or that you can only download stuff that youtube recompressed, but you'd be incorrect: there is a way to download the original content you uploaded to youtube.
Go here:
https://www.google.com/settings/takeout and once you do the request, they take all the videos you uploaded to your youtube account and create zip files that are up to 2 GB in size (for example if you upload a 1 GB video and 5 x 500 MB videos they may do 2 zip files, one with 4 videos (4x500) and one with two videos (1x1gb + 1 x 500 mb)
So this may be a nice trick of making a backup of your videos.