Now that I'm done, I'll create a very short overview of how I would approach this if I'd have to do it again:
Getting Forkcoins out of multisigInstall Electrum on an offline system, running from a LIVE Linux DVD.
Restore the 2FA wallet by creating a new wallet from your mnemonic, but choose to Disable 2FA.
Click File > Save a copy.
Open this text file in an editor. It'll give you 3 xpubs and 2 xprivs. This is what you need, but keep the xprivs offline!
Install a VM on your online system, install Electron Cash in there. Create a new wallet, select Multi-signature, choose 3 cosigners and 2 signatures. Import the 3 xpubs (NOT the xprivs!). This should give you a read-only wallet with the balance.
Create a legacy Bitcoin address any way you want. Make sure this is secure, preferably offline and air-gapped. Keep this!
Create a transaction from Electron Cash (your online watch-only wallet). Send all funds to your legacy wallet. If all funds exist on all chains (so your Bitcoins were all older than August 1, 2017), the lack of replay transaction means this transaction is also valid for BSV and BCH-A.
Click Preview, copy the transaction. Use a QR-code or USB-stick to copy it to your offline system.
On your offline system, install Electron Cash. Create a new multisig wallet. But this time it should be able to sign transactions, so use the 2 xprivs and the third xpub.
Open the unsigned transaction in your offline Electron Cash, sign it, and copy it again.
Use your online Electron Cash to broadcast the transaction.
Pray a bit, and calm your nerves
Check
https://blockchair.com/bitcoin-cash/address/1addy,
https://blockchair.com/bitcoin-sv/address/1addy and
https://blockchair.com/bitcoin-abc/address/1addy. If all went well, all funds on all chains should have moved from your multisig address to the legacy address on their respective chains.
Get some Forkcoin dust that only exists on one of the chains to ensure replay protection on all chains. If not all Forkcoins moved, you'll need to manually get them out of multisig again.
Use your favourite wallet for each Forkcoin to import the address (read-only) and create an unsigned transaction sending the entire balance (use all inputs on that chain, including the Forkcoin dust for that specific chain) to a new address on that chain. Sign the transaction on an airgapped system, then broadcast it. Repeat this for all Forks, start with the most valuable Forkcoin.
Bitcoin Gold and Bitcoin Diamond have replay protection, you'll need to get them out of multisig the hard way. BCD has a working Linux clone of Electrum, for BTG I used the Windows clone of Electrum using
Wine.
Disclaimer: use this at your own risk! Be sure to know what you're doing before doing it. When in doubt, don't do it. And don't hold me responsible, you and only you are responsible for following my advice.