My lower molar (tooth 19) had a large amalgam filling for 18 years. The
filling fell out a month ago. I never had any pain or problems with the
tooth prior to the filling falling out. Dentist says tooth needs a root
canal followed by post and crown.

Dentist removes decay and nerves completely from some "channels" but
says one channel has heavy calcification which makes the channel
impossible to navigate in order to remove nerve from that channel.

Dentists squirts something in there to soften it up, covers the tooth
with some kind of mastic and says in a week he'll try again with that
channel perhaps using some ultrasonic tool to break up the
calcification but that if he can't get the nerve out he recommends we
do the post and crown and hope for the best and that usually in these
cases the calcification will take care of that channel but that there
is a chance, up to 20%, that the tooth will have to be extracted anyway
due to leaving the nerve tissue in that channel.

Soooo, having already dropped $900 on this tooth so far and looking at
dropping around another $1000 for a post and crown I thought I'd
solicit advice here. Is this a good or bad gamble in your experience?
I plan to go ahead with the post and crown unless you guys/gals
convince me not to.