You might wish to ask this question on alt.autos.nissan.   This is a general motors
(chevy, buick, gmc, oldsmobile, cadillac)  newsgroup.      Not sure if the nissan
sentra was ever marketed under the GM banner.

Generally speaking,  if the battery light was on,  it means the battery is draining
and they determined the alternator to be at fault.     Did replacing the alternator fix
the battery light problem ???   However a defective battery can also cause an alternator
to burn out

You mentioned that you could hear the engine  "cranking",   well okay... that
might eliminate the starter  . (possible weak battery.... an internal high
resistance could have wiped out the original alternator and perhaps the
battery is too weak to crank the engine over quickly enough.  This is speculation.

Regarding the oil leak....

Have they  replaced the oil pan gasket already ???     Perhaps the oil drain plug was
somehow stripped and leaking from that point.    Placing a piece of cardboard
or newspaper under the front of the car would give you an idea of where the
leak was coming from.    

You might also wish to look  at the engine to see signs of oil leakage. (do this
with the engine OFF !!)

If the gasket and seal have already been replaced this is a moot point.

Suggesting what to do is difficult as I don't know the reputation of your nissan
dealer.  You might wish to have a second opinion from an independent shop.
Have them look at the battery,  including cables....    If you are located  by a Wal Mart,
they have a battery load testing machine which will tell you about the condition
of your battery.....  a major auto parts store might have the same equipment.

Then check battery cables very carefully,  including grounds.

I guess the car is at the dealer so the next quick check is of little value,  but if you
turn on the headlamps and then attempt to start car,   if the starter engages
(click click)  but the headlights dim,  chances are you have a defective battery
or high resistance in the cables.     Another way to test this out is to disconnect
the battery cables and use jumper cables to hook them to a good battery in
another car .... This will quickly prove if the battery is at fault and if the cables
are good......  car starts good cables,  bad battery.   Car doesn't start,  maybe
it is the starter  or you have high resistance in the cables (ohmeter time).

btw  www.advanceautoparts.com   has starters ranging from 109.00 to 170.00.

Regarding the CV Boot,  hopefully they caught it while it was cracking and
not ruptured thus saving you the cost of having to replace the CV joint or
half axle.

hope this helps a little but the more experienced people on this forum will
have a lot more valuable advice.

Peter

Okay, here maybe another possibility.   I had a 1990 Chevy with a 5.7
that refused tostart until I tapped the solenoid with a hammer.   After
that, it started without fail for one year (almost to the minute).   The
second time it happened, I pulled the starter and the culprit was - a
loose terminal on the solenoid.   I tightened the terminal and it's been
working great ever since (3 years since).   I'm not saying that this
applies to a Nissan; it just shows that little things can create big
problems.

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disatrous service  

Group: alt.autos.gm Date: Tue, Oct 17, 2006, 9:49pm (EDT-3) From:
zhuangqianli@gmail.com (conan)
About one month ago, I noticed the brake and battery lights on in my 97
Nissan Sentra (83Kmile). I brought my car to a Nissan dealer service
center. I also ordered a 3K mile oil change with 100 points coutesy
check. After the check,they replaced the alternator and the left outer
CV boot.
Right after that, my car started leaking oil, and there was an oil
burning smell in the car. I went back to the dealer several times. They
first insited that there was no leaking till last week, they
acknowledged that the " Oil pan gasket leaking" and " Front crank seal
seeping", but asked me to pay 500$ to fix.
But that was not the worst yet! Last weekend, 3 weeks after the service,
I could not start the engine. I could hear the cranking. Today, I had
car towed to the dealer. They told me to replace the starter, which will
cost 400$.
My questions: (1) Was the initial diagnosis correct? i.e. was the
symptom (the brake and battery light on ) really caused by the
alternator problem?
(2) was the oil leaking incurred by service (oil change, alternater
replacement)?
(3) is the starter problem related to the replacement of alternator?
The dealer denied all of these. What should I do on this issue?
My auto knowledge is almost zero. I would appreciate it very much if you
could me any help.
Conan  

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"Sometimes, when you're up to your butt in alligators, it's hard to
remember that the intial objective was to drain the swamp."  
~ Unknown ~