Assuming that there was no roughness when idling prior to the head gasket
blowing, it sounds as if the problem was introduced during the gasket
replacement process, so I would be leaning very heavily on the dealer. It
is, of course, possible that the leaking gasket was masking another problem,
but in my experience it can be difficult to track down such a fault on
modern complex engine management systems yourself - which is why dealers
have tech tools I guess. Also they are able to temporarily substitute
replacement components to try to eliminate the fault. That's not to say that
they haven't done something daft like left an intake hose or pipe not pushed
fully home or a hose-clip loose, or a vaccum pipe loose/split. It does
happen.  A good dealer really ought, with all the resources at his command,
to be able to identify and fix this. It's tempting to suggest trying another
dealer - but then you probably lose any leverage you might have with the
original one.
As for the turbo failing, it does depend on the service history and how the
car has been driven, with oil changes being particularly critical, but that
sounds very premature to me. If it definitely is shot, there are plenty of
options for replacement with rebuilt/reconditioned units. I'd be tempted to
go to a specialist turbo company. I wouldn't entrust the job to your current
dealer if he can't sort your other problems though.
Good luck!

The main problem you have is the dealer.  Unlkess this is warranty work
I would not be taking it there for these repairs.  Get the car to
someone that has a clue...

The Garrett turbos on the Saabs of that vintage are notorious for early
failures. I don't know if you have one, 2.3T sounds like a Mitsubishi
unit. If you can take a look at it you can tell easily.

Missfiring sounds like something odd on the crankshaft position sensor
or something related to the ignition... DI cassette? Perhaps when the
head was installed one of the timing gears was installed off by a
tooth. Why were the valves replaced?