Using an "AP Slough" ticket as example. It is clearly intended to be used on GWR trains but it doesn't have an operator restriction. If there is a disruption on GWR that it's impossible to travel on the booked service, on paper I'll be entitled to hop on an Elizabeth line train to my destination, is that right?
Depends on the route restriction. Some are "GWR & Hull Trains only", for example, in which case the only validity on the Elizabeth Line would be by dint of the Maltese Cross.
But yes, my understanding is any TOC restriction has to be stated in the Route field on the ticket, or it doesn't exist. And if it's "TOC X & connections" then where disruption means the passenger can't follow their booked itinerary, the connections could be on any TOC on any valid route that isn't precluded by the routing restriction. There simply is no rule to say that, where it is impossible to follow the original itinerary, you have to stick as much as possible to the same route / TOCs as on the original itinerary. In my view you can take whatever is the best/fastest available route.
You do have to stick to the route and TOC restrictions in the Route field on the ticket (as opposed to on the original itinerary), unless ticket acceptance is in place or you've sought and obtained permission to take a different TOC's train and/or travel by a route that wouldn't normally be permitted. The only circumstance where I would ignore the TOC or routing restriction without gaining permission first is where I would otherwise be stranded and there was no opportunity to get prior permission, but even then I'd be tweeting the relevant TOC(s) to explain the situation (including that there was nobody to ask before I had to board) and to ask for assistance, and I would seek out on-board staff rather than wait for a ticket inspection.