I don't know enough about js and the app itself to be sure what would need to be changed. You would also have to find were the /addr is located and change it to /address and a few other things, but I don't see it as an impossible task. Just a long and annoying one.
-Dave
I could potentially make the change you are suggesting to the js file and I am not necessarily opposed to long and annoying (you have no idea how much work I have put into this already). Unfortunately I am a terrible programmer and cannot read or write code well at all and I don't trust myself not to screw it up. On top of that, this is only step 1 and I don't even comprehend steps 2 or step 3-a or step 3-b
I was under the impression that Coinbase's multisig vaults have long been discontinued and they removed all support for them, and so this will no longer be an option. But regardless of what I think - have you tried this? It seems like it would solve all your problems if it is possible.
If you do not have the decryption key (and the above is not possible), then your options are limited to either brute forcing the decryption key so you can use the user seed and shared seed, or seeing if Coinbase will give you access to the Coinbase seed linked to your account (which seems highly unlikely).
I can see my vault on coinbase and when I click "withdraw from vault" nothing happens.
I am not certain that I do not know the decryption key for the shared seed. Unfortunately, there isn't any way to "test" it.