[Ethereum] What Hierarchical Deterministic (HD) wallets are available for Ethereum

There are several bitcoin standards related to creating keys from mnemonic seeds.

BIP 39 for making mnemonics to create a seed: https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki

BIP 32 for deriving keys from that seed: https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki

However, not all clients use all these standards (eg Electrum's mnemonic implementation is different - I think it pre-dates the standard and the developers weren't really happy with the standard) and there's a little bit of room for different ways of implementing things even if you're trying to follow it; For example, a wallet is allowed to have different derivation paths for different "accounts", but it doesn't have to.

So no, you can't quite be confident your seed will still work as is on an arbitrary future wallet, although it might. But if there's a lot of money in there it should be possible to dig out the way the old wallet was doing the thing and re-implement enough of it to get your private keys out.

Did you check the words you wrote down by re-entering the phrase into the phone at the time you made the copy? Otherwise, you might have made the same mistake several times (less problematic with BIP39 passphrases than alphanumeric codes where 1s and ls, 0s and Os look similar, but still possible). You should have 12 English words as part of your phrase. If you do count 12 words, make sure that all of them come from this list. If they do not, then you can try using words that are close to the ones you wrote down. If you did, in fact, test your phrase, then perhaps there was a bug fixed or introduced at some point between when you installed the app and now. In that case, things should work if you install an older version of the app onto your phone.

Decentral, the creator of Jaxx, does not store keys for Jaxx accounts which means that you are the only holder of the keys to the account. Also, there is no central authority for Ethereum (despite what the Ethereum Classic folks might try to tell you) who can help reverse the transaction or otherwise recover the ether. Thus, if you do not have the keys, if Ethereum is working as intended, the ether is lost to time.

Going forward, it would be wise to follow best practices for backing up if you have any sizeable holdings of Ethereum -- you are your own bank! This means things like following 3-2-1 and test-restoring your backups regularly. I would go further than the link suggests and not only use different media, as they define it, but also different formats: SSD, SD card, tape, or even an entirely functioning device such as a phone to ensure that you are always able to read your data and that obsolescence, firmware bugs, or manufacturing defects in any one medium don't result in data loss.