POAUSDT
POA / Tether USD
crypto

I think it's unlikely this would trigger an official POA designation but certainly I'm acting as their legal agent with respect to management of these funds

Is this some sort of you acting as their agent? In that case, get this written down, and get parents SS numbers for opening accounts.
Most states have a statutory POA form, the brokerage will want to see it. Example https://saclaw.org/resource\_library/power-of-attorney-california-uniform-statutory/

POA doesn't factor into it at all. If it's a TOD brokerage account, normal inheritance / step up basis rules will apply.
POA will nullify upon their death, but that's not what you're asking I don't think.

Why is your mom trading? She shouldn't be actively trading unless she understands what she is doing. If you have POA on file with fidelity go into the branch yourself.

If you are gambling in her account with limited authority they may feel that the risk goals on the account (conservative / income) don't match the trading decisions or feel that the account owner may not understand the trading activity? If she does understand, have her call in and explain she understands what is going in the account.