John Doe wrote:
I terms of depresurization, if there are truly no operable valves to
depressurise a progress in space, couldn't they just use a space hammer
and a long nail to punch a hole in the pressure vessel ?


As soon as the air starts shooting out, the Progress will start moving
around from the thrust of the escaping gas.
You might be able to store stuff in the unpressurized section of the
Progress that replaces the Soyuz reentry module though (the pressurized
section is a modified Soyuz orbital module), but it's not designed to go
into that orbital inclination.
So you would have to give the Russians a lot of advance notice (months)
to even attempt it.
On the other hand you could stick a grapple fixture on it for the
Shuttle's RMS to grab and lower it into the cargo bay after rendezvous.
But payload would be very limited to that high of a orbit at that low
of a orbital inclination.
We're pretty much counting on Atlantis working right or a very quick
turnaround for pad 39A.
Worth it?
Hard to say.
Recent Shuttle flights have been going very well as far as ascent damage
goes.
So the HST repair mission's overall risk is pretty low, but certainly
not nonexistent.
On the other hand, HST is getting long in the tooth and needs
replacement...the recent failure of the communications system luckily
occurred a few weeks before - not a few weeks after - the planned repair
flight.
It really does need to be replaced with the new generation Webb Space
Telescope*, as we can learn a whole lot from having operated HST over
the past 20+ years and its basic technology is getting pretty dated,
having come from the late 1970s-early 1980s.
For starters, we can grind the mirror correctly next time around.

* http://webbtelescope.org/webb_telescope/

Pat