Presumably a reasonable number of passengers *south* of Wealdstone want
to go places other than Euston (as Wealdstone was chosen as the
truncated Bakerloo terminus), so running only Silverlinks north of
Queen's Park would be pretty unpopular.

This would be mitigated by the easy cross-platform interchange at
Queen's Park. Although I accept that some people hate changing trains
regardless.

A shuttle would probably be pointless given the availability of
alternative stations at Kilburn Park and Swiss Cottage with a much wider
range of destinations.

I was thinking also of keeping Euston available as a destination from
north of Queens Park.

Some services from Watford might be diverted down the West London Line.

Would they be able to stop at Willesden Junction?

Not given the current infrastructure, but it would be possible if some
extra platforms were built...

It does seem a bit pointless though, just so that they can run through
trains without people having to change at Willesden (albeit a faily
inconvenient one from LL to HL). IMO the service pattern could do
without being made more complicated. A decent Willesden-Clapham
frequency is what's needed more than anything.