Can you remove the locking ring? Worth a try.
Don't know what an A15 bulb is but the overall with the bulb I have porcelain to end of bulb is 3 1/2". See image
It occurs to me that Rob and Stuart were giving the overall length of the adapter, and not considering that the screw-in bulb actually fits inside the adapter for some depth. The first answer from niceyard is probably the correct answer, as niceyard actually gave the overall dimension with bulb installed.
I bought mine in California store.
No ,but you can buy the bulb with that feature.
I used mine on an outside garage door.
Assuming you mean a "100-watt equivalent" LED, of course you can.