My sister claims they could see it in some separate "legacy wallet" showing up on the trezor interface?
I don't own a Trezor zo I can't verify this, but can you check: Does the address she sent you start with a "1"? And do the other addresses that were used on the Trezor start with something else?
13. When questioned where they acquired the address from that they sent me... they are both very certain that they got it from their trezor.
It could be as dumb as
copy paste malware that could have changed the address when creating the email.
I wondered if I could build and submit a tx to the blockchain requesting the transfer of the coin from that address to one of my addresses and somehow provide the private key (The mnemonic seed phrase they gave me) as my credentials for having the authority to spend the coin and would the blockchain move the coin if indeed the private key was correct?
If the private key is correct, you can sign the transaction and broadcast it. You can't "negotiate" with "the blockchain" to send your coins in any other way.
You could play around with
Ian Coleman's Mnemonic Converter (do this offline, airgapped on a Live Linux boot running from RAM), play around with the derivation path, and see if the addresses it spits out match the original. But even though I don't have a Trezor, I don't think it's very likely to spit out addresses that can't be restored. In fact, that should never happen as long as the address was verified on the device's screen.
She's not asking me to replace it
Well, she shouldn't: as long as your payment arrived on the address she provided, you did your part.
Any other questions I should grill my sister on?
I'd start the interrogation with this:
some separate "legacy wallet" showing up on the trezor interface
I did put the 2nd seed phrase into an electrum wallet of my own and it does indeed show all their tx history and does cover the time frame in which I did send them the coin. So I believe the 2nd seed is the correct one. But the electrum wallet is indeed empty. So I am baffled as to what they might have done? As in where they got the address from?
Don't forget to check if there are any Forkcoins left on the addresses.