Stable enough for Apollo 11 maybe for its short duration of a couple of hours in the surface but not long enough for Apollo 17 that was at Taurus-Littrow for three days.,
Also, for Apollo 13 there was a lot more communications with the spacecraft in transit (and remember that the Apollo spacecraft in transit to and from the moon goes a long way off the a direct line between the Earth and the Moon)
It depends on the amount of instability at the L1 point, I know it is one of the least stable of the Lagrange points, beyond that I'm guessing. But as for tracking, that could be done from each of the Saturn launches.. But it just goes to show, all these little intricacies and difficulties, it was simpler and easier for it to have happened..
Back of the envelope math.
L1 is about 325,000 km from earth, the moon is about 384,000km, so L1 is about 85% of the distance. While you could possibly put a relay station there it wouldn't help much. If you are going to fake the landing site on Earth, say, at Area 51, then in order to make the signal come from the direction of the moon (to anyone listening), you still have to get the signal from
CapCom to L1 - 325,000 km
L1 to SSTN* - 325,000 km
SSTN* to Area 51 - ?
Area 51 to SSTN* - ?
SSTN* to L1 - 325,000 km
L1 to CapCom - 325,000 km
*SSTN = Sooper Seekrit Tracking Network
In 1969, the signal to and from Area 51 and SSTN would probably be achieved by landline and the delay would be typically 1-2 seconds each way (Others here will have a better idea of that than me)
The signal has to travel through space 1.3 million km
1.3 million km @ 300,000 km/sec = 4.3 seconds plus 2 to 4 seconds landline delay means that even an immediate reply to a transmission from CapCom could not be heard any quicker than 6 to 8 seconds later.
That delay would stand out like a canine's gonads!!!
NOTE: With 1969 rocket technology I think it would be bloody difficult to put a satellite at L1. As I understand it, the Apollo missions were all done by, as Tom Hanks said in Apollo 13,
"putting Sir Isaac Newton in the driving seat". The spacecraft literally fell toward the moon and they used the moon's gravity as well as retro-firing to put the the spacecraft into lunar orbit. The orbital mechanics experts here will have a better idea than me, but I imagine it would take quite a complicated series of manoeuvres and extended lunar orbits including a number of retro-firings to get a satellite into L1.