I really think you are technically minded enough to sort out your problem and you already said that you had been using many possible wallets for your client which is then correct because I really think we should have multiple wallets if we would want to hold for more coins than we can ever carry, And next is having a 3rd party that both of you and your clients trust, This will be the sole purpose of it because you need someone to sure verify and secure your wallet and an attorney is simply there for the legal agreement you and your clients will have, And lastly, In my opinion, I think you can then use regular wallets for your convenience and I think that even though there is a strong security wallet that you may found there is still a possibility that it may still compromise because of the upgrading technology we have hackers may find themselves upgrading as well.
This forum is full of people who can't read posts but feel the need to comment just to make themselves feel knowledgable.
