Your phone (primary device) and the linked ones have to share the IK since that is the "root of trust" for you account: with that you generate new device keys, renew them and so on.

Those keys are backed by Keystore on Android, and some similar system on Windows/Linux, i'd assume the same for MacOS/iOS (but I don't know the details) so it's not as simple as just having access to your laptop, they'd need at least root.

Phishing is always tricky, probably impossible to counter sadly - each one of us would be susceptible at the wrong moment.

I think the point is that as a user you expect revocation of trust to protect you going forward, yet it doesn’t (e.g. the server shouldn’t be forwarding new messages to). That’s a design decision Signal made but clearly it’s one that leaves you open to harm. Moreover, it’s a dangerous decision because after obtaining the IK in some way (e.g. stolen device) you’re able to then essentially surreptitiously take over the account without the user ever knowing (i.e. no phishing needed). As an end user these are surprising design choices and that Signal discounted this as not part of their threat model to me suggest their threat model has an intentional or unintentional hole; second-hand devices that aren’t wiped are common & jail breaks exist.

This isn’t intractable either. You could imagine various protocols where having the IK is insufficient for receiving new messages going forward or impersonating sending messages. A simple one would be that each new device establishes a new key that the server recognizes as pertaining to that device and notifications are encrypted with a per-device key when sending to a device and require outbound messages to be similarly encrypted. There’s probably better schemes than this naive approach.

Revocation of trust is always a tricky issue, you can look at TLS certificates to see what a can of worms that is.

The Signal server does not forward messages to your devices, and the list of devices someone has (including your own) can and has to be queried to communicate with them, since each device will establish unique keys signed by that IK, so it isn't as bad as having invisible devices that you'd never aware of. That of course relies on you being able to ensure the server is honest, and consistent, but this is already work in progress they are doing.

I think most of the issue here doesn't lie in the protocol design but in (1) how you "detect" the failure scenarios (like here, if your phone is informed a new device was added, without you pressing the Link button, you can assume something's phishy), (2) how do you properly warn people when something bad happens and (3) how do you inform users such that you both have a similar mental model. You also have to achieve these things without overwhelming them.

I would be surprised if there aren’t ways to design it cryptographically to ensure that an unlinked device doesn’t have access to future messages. The problem with how Signal has designed it is that is a known weakness that Signal has dismissed in the past.