spark plug break off, internal

i have an 04 f150 4x4. i was getting a rough idle and
coughing and backfire under acceleration. i took it to my
mechanic and we changed 02 sensors. codes read lean and
cylinders 4 and 8 misfire. he said he found a leak in intake
so he took it off and replaced intake gasket. he said he
found a spot where it was leaking. the truck ran great for a
few miles, 5 to15, then would start chugging and puking and
backfiring again. lost power every time. i could pull over and
let idle for 30 seconds and run good again for a few miles.
he did a compression test and when he pulled out sparkplug
on #8 the bottom of spark plug was missing. can't find it.
the compression on #8 was 20#. we suspect it broke off
and went the the exhaust valve. i tried to drive it home and it
had rougher idle then ever and would die at stoplights. i
would have to jump start it to get it going. seemed like
something was in a bind. (hard to turn over). then i couldn't
get igt to turn over at all.  made it halfway home and had to
tow it the rest of the way. any thoughts? is motor blown?
should spark plug manufacturer be responsible for repairs?
HELP