I'm completely confused by this, unable to make anything I've read on
the internet really fit.

Lower left molar with large filling and a cusp fracture dating back a
few years, had no money to repair it due to other medical expenses.
(Not mine.)  Finally became infected (or clinically evident infection)
about 6 weeks ago.  X-rays showed deep decay, but both the dentist and
endo agreed it could be saved.

Pre-RCT pain was exquisite when trying to sleep, and could be
periodically inflamed during the day by anything from cold or hot drink
to certain mouth movements that created suction.  Tooth was sensitive
simply to light touch at some times.

Five weeks ago - endodontic appointment with a local group of
endodontists, supposedly the best in the city.  Had trouble getting the
tooth numb, took quite a bit of time and anesthetic and trial and error
with pain.  Endo said it was "hot."  Once it was fully numbed, the
procedure seemed to go well.  Endo has all the latest gadgets,
microscope, rotary drill, etc.  Placed a post and did a build-up for
the crown.  Post-RCT pain was minimal except for tenderness to contact
on the buildup.

Crown was placed two days later.  Tooth was still tender, but nothing
too bad.  Did a bite test that seemed largely okay.

After several days of staying off of the crown, I noticed when eating
(by accident) that I was able to chew on it.

I soon after, however, noticed that any jiggling movement elicits a
feeling of tenderness, almost soreness.  This happens if I attempt to
"wiggle" the tooth, or with percussion, or sometimes when chewing if
the food pushes laterally on the tooth.

I can actually chew (straight up and down chewing force) with no
discomfort at all.

There is no pain upon releasing bite pressure.  (Which I understand is
the hallmark of a fracture.)

The tooth does not hurt on its own.  It is not sensitive to heat or
cold or palpation.

The tooth does not feel loose, does not wiggle that I can tell.  It
feels pretty solid.

The tenderness varies throughout the day.  Sometimes it gets bad enough
that the force of my tongue brushing it can elicit a sensation of
tenderness.  Not shooting or stabbing pain, not even pain, just clear
tenderness.  It has only gotten this bad after a lot of chewing on that
side one day of some pizza that had rather firm crust.

Sometimes the tenderness is all but entirely gone.  The other day it
was gone when I awoke, but reappeared after a few hours even though I
had eaten nothing at all.

The dentist who placed the crown has shaved it down a bit once, but he
said it looked like there was just one spot where there may have been a
"little" contact, it did not sound as though the bite was very off.
Shaving it down did drop the frequency and degree of the tenderness,
but as far as I can tell now the tooth is not touching the opposing
teeth at all yet the tenderness remains.

My biggest fear is a fracture that could cause the tooth to need
extraction.  I am in my 30s, have all of my teeth, but when the topic
of possible extraction of this tooth was originally raised my dentist
seemed hesitant, citing something about compromising function given
that I had several teeth extracted when I was young for orthodontic
reasons.

I suppose my questions are:

1.  Does this sort of pain sound like a fracture?  Again, there is no
pain on release of bite pressure, and it takes only the lightest
percussion to generate the tender sensation.  I was under the
impression a fracture generally caused pain by allowing the tooth to
flex, but this light percussion wouldn't seem to be enough to cause any
flexing.  Only lateral movement and percussion create discomfort, and
the lateral movement also does not create additional pain on release.
For example, if I press the tooth forward there is tenderness but then
it stops if I hold the tooth in that position, and there is no pain
when I release it.  Same for pressing the tooth in a different
direction.  And no pain at all pressing the tooth directly down.

2.  It certainly feels like perhaps the ligament is inflamed, as it has
all the hallmarks of a 'squishing' sort of tenderness.  Bite pressure
on this tooth probably presses it backward a bit, and it's my
impression that pressing the tooth backward manually elicits a lot more
tenderness than in other directions.  Is it possible this is the
problem, even after 5 weeks?  Note that I do not press and wiggle on
this tooth constantly, I've only done this to try to assess the nature
of the tenderness.  I've even given that side entire days off from
chewing and have worn half of a night guard on -- on the opposite side
-- to try to limit unconscious clenching throughout the day or night.

3.  Is it common for the endodontist to test for fractures during a
root canal w/microscope?  Is there some sort of standard staining or
other process?

4.  If this was a periapical infection, would its intensity vary this
way?  Would there be other signs?

X-rays were taken of the tooth after the root canal, and after the
crown placement.  The endodontist (an older man with a great deal of
experience) said he thought it turned out great, and the dentist (an
LVI guy) seemed to concur.  No one has mentioned any sign of infection
or other issue with the tooth.

I simply don't have the money to spend for a lot of additional work
given the extreme financial hardships of my wife's cancer, and I'm
trying to make sure I'm as informed as possible before making any more
decisions.  I have an appointment with my dentist next week to revisit
the issue.  I like this dentist, I have no reason to distrust him, he
has actually been very helpful, but in the years of our struggle with
this cancer I've learned there is no better advocate than an informed
patient.

Any help would be appreciated.