A couple of years back, my DMO dentist did a composite filling on the
furthest back upper molar (right side). Before I had him do it, there was
some hacking back and forth with the insurance company. His office first
said my insurance plan didn't cover composite fillings and he only did
composites on that tooth. Turns out yes, it was covered and they ended up
doing it.

Since then, they've got me on a quarterly perio maintenance cleaning (after
hitting me for $500 worth of scaling, root planing, arestin at $40 a pop -
meds aren't covered under my plan).

After my annual check up, he announces that the tooth that got that filling
now has a cavity in the area above the gumline, where it butts up to the
next molar, just below the filling (that he did). He showed me a dark spot
on the x-ray that is the decay.

What gives me cause to wonder is that between daily brushing and vigorous
daily (more or less) flossing, those perio maintenance cleanings 4 times a
year for the last year and a half or so, my teeth have been getting more
cleaning attention than they've ever had (and only had 2 cavities prior to
all this which only appeared within the last 10 years) which leaves me
puzzled how this cavity could even have a chance to develop. I've also
completely eliminated colas since I've been told the phosphoric acid likely
is largely to blame for the 3 previous cavities I had, which were all in
rear molars at the gumline.

Some questions:

1) Is this cavity something that should have been detected during perio
maint. cleanings? He says no, that since it's between the teeth and small,
it's about impossible to spot without an x-ray.

2) With the quarterly cleanings/brushing/flossing is it still plausible to
suddenly develop a cavity like this?

3) He says a crown is the only viable option because of the amount of tooth
area that's damaged from the previous cavity/filling which runs along most
of the lenth of the tooth right at the gumline.  Their "customary charges"
for a crown are around $1200, with insurance it's going to be $500 out of
pocket. I always seem to get soaked for $500 with this guy.

4) If the cavity developed just under the line of the filling, is it
possible it's because of the way the filling was done? I.e. - could it be
his fault? For that matter, the more paranoid portion of my mind wonders if
a dentist who is so inclined can deliberately put in a filling in such a way
as to cultivate a cavity.  "Okay mofo, you get out of paying me full retail
for the filling now, but I'll getcha in a year or two with a crown when this
bad boy develops another cavity".  Or is that just ridiculous?

Thanks for all input.