Some commentators in this post have questioned why the article is called what it is when it is just about HS2. For me I think that the title is apt and I now have so much more clarified. This is a really good article and I finally understand and like the project. Before I didn't give a damn about it.
As for not planning another exit route out of Euston, I'd imagine that there's a significant planning headache in routing new tunnels around the existing infrastructure in and around Euston, and that having the line shadow the existing WCML is much less painful to both plan and develop.
Paddington has had a significant amount of work done to it recently with the Crossrail work going into it. Though it doesn't appear to suffer the access issues that Euston has it may prove to be a headache if HS2 traffic was supposed to arrive alongside GWML traffic.