- once a Seed is offline created, can I go online with it (using Electrum on Tails)
or would that defeat the purpose of offline seed creation?
If an attacker manages to steal your wallet (via malware) file and your password (via keylogger), it won't matter that you generated the seed offline. So imho it kinda defeats the purpose.
On the other hand, the attacker needs to get both - wallet + password -, hence offline seed could be a tiny bit safer.
However, if you already know how to use Tails, I suggest you keep that wallet on Tails 100% offline, as cold storage, and keep on the live/online computer a watch-only wallet.
And just read and follow the cold storage docs:
https://electrum.readthedocs.io/en/latest/coldstorage.html- do you overwrite your Electrum-wallet backups on the removable offline storage media
or shoud different backup-version be stored (e.g.: wallet-bu_2021-04-01, wallet-bu_2021-04-06)?
I don't have this problem, I only backup seed(s). Unless you do anything fancy, seed should do.
If you keep files, I'd keep at least the last 2 versions, just in case one of them is saved damaged for whatever reason - it's a rule I follow for all file backups I make, no matter where from.