So let's get this right.... You're proposing that we spend a fair chunk of the money allocated to HS2 (remembering that the underground station at Euston and the tunnels leading out of it are going to be one of the most expensive parts of HS2, because building underground stations is expensive). But instead of getting an entire new line giving up to 18tph of long distance trains and massively relieving capacity on the WCML, you want to use the massive expense of this new underground station and approaches to basically get maybe a couple of extra tph on the Chiltern line and divert a few trains from the Great Western Line (I'm assuming you meant the GW line not the West Coast ML)?
That's similar to what I'd do.
Scrap HS2, keep the Euston/Camden regen as that's pretty advanced. Not one penny wasted on a new railway line but still benefits.