Somebody poke holes in this. What's the benefit of multisig? What am I not seeing?
A single point of failure.
The main benefit of multi-sig over single sig plus passphrase is that multi-sig does not have a single point of failure. You can use three different devices to generate three different seed phrases, and only move the xpubs between devices in order to generate addresses. The compromise of any one device does not lead to compromise of the wallet. The same is true when spending from a multi-sig - you can keep each set of private keys on separate devices, and so one compromised device never has enough information to steal the coins.
With single sig plus passphrase, you
must bring the seed phrase and the passphrase together on the same device both to create the wallet and also to spend from the wallet. If that device is compromised, then your funds are also compromised. The best way to address this is to use either a permanently airgapped computer running an open source OS, or use an open source airgapped hardware wallet such as Passport.
In terms of back ups, then the security and redundancy is comparable between a single sig plus passphrase with two back ups of each component and a 2-of-3 multi-sig. In both scenarios you can lose one back up and still recover your wallet, and in both scenarios an attacker needs to compromise two back ups to steal your coins. The single sig plus passphrase is actually slightly safer in this arrangement since you might be able to lose two back ups and still recover your wallet, and an attacker might need to compromise three back ups to steal your coins, depending on which back ups are involved. The down side is you need four back ups instead of three.
I'm trying to game out how to explain xpubs and descriptors and derivation paths to people who aren't technical.
You shouldn't need to explain either descriptors or derivation paths. Back up your three seed phrases along with one xpub, so the recovery of any two back ups provides two seed phrases plus the third xpub, as follows:
Back up 1: Seed A, xpub B
Back up 2: Seed B, xpub C
Back up 3: Seed C, xpub A
Then your family member can simply follow the instructions on a wallet such as Electrum to recover the multi-sig.