Multisig account optin: no onchain trace?

For single sig account I have opted in, I see a transaction on the nem blockchain referencing the corresponding symbol account public key. But for a multisig account I opted in through the cosignatories, I don’t see a transaction on the nem blockchain. How can I validated that the multisig account is opted in (without relying on the wallet), and can I possibly get its public key from the nem blockchain somehow?

It’s pretty same for multisig.
First all cosigners need to opt-in (single opt-in).
Next you initializing from one of cosigners opt-in for multisig (this transaction contains details you search) and all other cosigners need to confirm.
If you have some multisig account that already opt-in I can describe on example

I took another look. In the wallet, from a co-signatory account, I see that the multisig account is opted in: I see the balance opted in, and cosignatories having validated the optin transaction. I also see a transaction from the cosignatory referencing the public key of the multisig symbol account to be created, but I do not see any transaction from the multisig account regarding optin. Is that normal?

Yes. It’s normal. It’s not NEM multisig transaction. Only transaction sending here cosigners.