Are you looking for someone to handle the actual TPA administration (compliance testing, contribution calculations, annual reporting, etc) or someone to strike a daily price for a unique portfolio (similar to a collective trust account)?
I don't really care too much either way, but using the same password for more than one account any more is very silly. Use software like Lastpass and have a single, very complex password for everything. And use two factor authentication when available.
Then the user id won't matter much.
This is actually some progress, but I would take the additional step of having your new atty contact Northrup and Fidelity with a formal request. That usually helps move things along.
But let's still assume that they are legally unrelated for tax/retirement plan purposes. If I'm a board member on Org B and not on Org A, I would be very concerned.
If organization B isn't controlled by the same board as organization A, why would the board of org B agree to do this? And aren't they violating their duty to org B?