only it leaves mints in a very tenuous legal position
That's not a good reason to pretend they are something else. You don't get to make up new definitions just because you don't like the legal consequences of the correct definition.
and hides the fact that you do have some power that traditional custody doesn't afford to you
Then it's "enhanced" custody...but still custody. Imagine if every custodian who added some new feature declared that they weren't a custodian anymore because "traditional custodians don't have feature X!" It would be ridiculous. If you still have user funds, you're a custodian, and just because ecash mints have some cool features does not make them non-custodial.
While I'm here I'd like to comment on a few sentences from your opening post:
Most people in the Bitcoin world understand custody to mean that someone else holds the keys to your bitcoin and all you have is an account balance
I think it's just the first part, before the word "and." Being custodial has nothing to do with how they represent their relationship to you in their database. For example, the website acceptln.com is a custodial website that does not have accounts. Instead, you "deposit" sats without logging in and it automatically emails a "voucher" for that deposit to whoever you want it to go to. They email that person a link where they can claim the sats. Here, the depositor never has an account, but it's still obviously custodial -- acceptln has the money. What makes you a custodian is whether you have the keys, not whether you give your depositors an account or not.
[mints] are fundamentally different from this common conception of custody because users do not have accounts with the mint
That does not matter. The mint holds user deposits and redeems them when bearer tokens are submitted to them. I like the analogy of a money order. If I go to my local walmart I can purchase a money order from them which is essentially a check drawn on walmart's account. I need no account of any kind to do this. I can then mail this money order to anyone and if they go to their own local walmart, or walmart's bank, they can redeem it, because it's walmart's check. Ecash tokens are a lot like that. But it's clearly custodial. I give my money to w̶a̶l̶m̶a̶r̶t̶ the mint, they keep it but give me an IOU for the same amount (minus a fee), I can give that to anyone I want, and they can redeem it without an account. But since w̶a̶l̶m̶a̶r̶t̶ the mint has my deposit the whole time, w̶a̶l̶m̶a̶r̶t̶ the mint is the custodian.
users are able to trade their ecash tokens with anyone they wish without the mint's knowledge or permission.
Not if the mint doesn't want you to. The problem is the melt/swap operation. If you send someone your IOU and the recipient doesn't swap or melt it at the mint, the IOU is double spendable -- you can give it to someone else or redeem it yourself. The recipient hasn't really "received" anything of value if it's still fully spendable by whoever they got it from. Therefore, ecash recipients have to swap or melt their tokens in order to safely consider them "received." And that means the whole system is permissioned, because the mint is under no obligation to issue IOUs for anyone and they are under no obligation to redeem them for anyone. They can refuse, or do shotgun KYC, or suddenly raise their fees, or do whatever they want, at any step of the process. They can suddenly start doing this at the time of issuance or at the time of redemption or anywhere in between. They hold the money so it is in fact a permissioned system. Which is just another indicator that it really is a custodial relationship, regardless of whether users have accounts or not.
That's not a good reason to pretend they are something else. You don't get to make up new definitions just because you don't like the legal consequences of the correct definition.
Then it's "enhanced" custody...but still custody. Imagine if every custodian who added some new feature declared that they weren't a custodian anymore because "traditional custodians don't have feature X!" It would be ridiculous. If you still have user funds, you're a custodian, and just because ecash mints have some cool features does not make them non-custodial.
While I'm here I'd like to comment on a few sentences from your opening post:
I think it's just the first part, before the word "and." Being custodial has nothing to do with how they represent their relationship to you in their database. For example, the website acceptln.com is a custodial website that does not have accounts. Instead, you "deposit" sats without logging in and it automatically emails a "voucher" for that deposit to whoever you want it to go to. They email that person a link where they can claim the sats. Here, the depositor never has an account, but it's still obviously custodial -- acceptln has the money. What makes you a custodian is whether you have the keys, not whether you give your depositors an account or not.
That does not matter. The mint holds user deposits and redeems them when bearer tokens are submitted to them. I like the analogy of a money order. If I go to my local walmart I can purchase a money order from them which is essentially a check drawn on walmart's account. I need no account of any kind to do this. I can then mail this money order to anyone and if they go to their own local walmart, or walmart's bank, they can redeem it, because it's walmart's check. Ecash tokens are a lot like that. But it's clearly custodial. I give my money to w̶a̶l̶m̶a̶r̶t̶ the mint, they keep it but give me an IOU for the same amount (minus a fee), I can give that to anyone I want, and they can redeem it without an account. But since w̶a̶l̶m̶a̶r̶t̶ the mint has my deposit the whole time, w̶a̶l̶m̶a̶r̶t̶ the mint is the custodian.
Not if the mint doesn't want you to. The problem is the melt/swap operation. If you send someone your IOU and the recipient doesn't swap or melt it at the mint, the IOU is double spendable -- you can give it to someone else or redeem it yourself. The recipient hasn't really "received" anything of value if it's still fully spendable by whoever they got it from. Therefore, ecash recipients have to swap or melt their tokens in order to safely consider them "received." And that means the whole system is permissioned, because the mint is under no obligation to issue IOUs for anyone and they are under no obligation to redeem them for anyone. They can refuse, or do shotgun KYC, or suddenly raise their fees, or do whatever they want, at any step of the process. They can suddenly start doing this at the time of issuance or at the time of redemption or anywhere in between. They hold the money so it is in fact a permissioned system. Which is just another indicator that it really is a custodial relationship, regardless of whether users have accounts or not.