According to enquiries, the download code supplied here has no time restrictions and you can therefore still redeem it in December, for example. :)

Yes, it will be a download code and with that you get 12 months of Nintendo switch online or do you mean until when you can redeem it?

Generally, this membership should only become active after the old one has expired. So you won't be able to have a "double" membership, if I'm not mistaken.