If I have to use a multisig online wallet the only things I ask for are:

in a 2 of 3 multisign I have to create the 3 priv keys  and I am the one who gives one of them to the online wallet.

the other 2 will be cold priv keys  ( one on a trezor/coldwallet and the other on a paperwallet)


privs keys generated for third-parties ( or even online created ones) are not enough for a serious user.

IMHO is better an aproach like this:
https://api.trustedcoin.com/#/example-2-of-3