It sounds nice but I'm afraid I still can't see that working. Let's say you have somehow found the money to build a flyover at Watford Junction to get the service onto the LNWR lines at Watford Junction without causing conflicts there, and you've also found an extra path on those lines from Watford to Euston, with sufficient slack to stop at all those stations without delaying the faster LNWR services. Now you have the problem that you're going to want to run at least 8/10 car trains because anything less going into Euston would be a complete waste of badly needed capacity. That means more work on the Abbey line to lengthen the platforms. And are there enough passengers on the Abbey line to run trains that long? Well, I agree that through trains to London would attract more passengers, but if
@Bald Rick's statement that current passenger levels would fit on a bus is correct, then you'd need a LOT more passengers. The trouble is you're still only running an hourly service, and that's never going to make the line attractive. AND by running to London, you're potentially importing delays from the WCML onto a what's basically a 6 mile long siding that has little ability to cope with any disruption.
I think you'd get much greater benefit from keeping it a separate line but adding the passing loop necessary to run a half-hourly service and get more passengers from having a more attractive frequency instead.