First floor shower leaking into the basement

[Underdog]

Hi, first post. Thanks for welcoming me.
My sister's 2 boy's are home from college and her shower/bath is leaking.
The shower leaks about 6 ounces in 15 minutes into the cavity where the access panel is. No leak when the water is running into the tub. The plumber quoted her $1500 to fix the leak but he can't get to if for at least a few months. She just wants the leak to stop for now.
I opened up the access panel in the adjacent closet and can see the leak.
With the shower running the water is dripping but when I place my hand on the what looks like a diverter valve assembly (integrated into the tub spout) everything is dry. The water is only dripping from down lower where the surround meets the old cast iron tub. I thought for sure it was a leaky panel or bad caulk joint. But I taped plastic shower curtains all around the walls of the shower stall and ran the shower only to find it leaking exactly the same as before.
Where could the leak be coming from if not from inside the stall? Could the spout/diverter valve leak from somewhere closer to the surround panel?
Note: up at eye level (top of shower stall) the shower head has a threaded chrome stub that leads out of the wall to the showerhead.

 

[Mr_David]

take a cup and pour water on the caulk joint around the tub with out running any water from the tub valve. repeat several times. That is way to much caulking. More is not better. Could even be from the vertical seam to the right.

There other possibility is the other end of the nipple inside the tub spout . Just unscrew the spout and have a look see.

when you divert the water it puts back pressure on the nipple the spout is screwed on to.

 

[Underdog]

So am I to understand that when the diverter is pulled it puts pressure on the connection between the inside of the spout and the nipple that it is attached to. Is there a plastic insert in the spout (that was screwed on the nipple) that breaks? Would this allow water to to push back towards the wall (through the the surround) and leak out behind the tub surround panel? Would that explain why the leak only occurs when the diverter is pulled on the spout?

Does this happen only with "threaded on" spouts and not the type of spout that slips on and is secured with an allen wrench?

[ame="https://www.youtube.com/watch?v=DlrEDWUacLg"]https://www.youtube.com/watch?v=DlrEDWUacLg[/ame]

 

[frodo]

looks to me like the nipple is to short by about a 1/4'' or so. on the spout .

looks like the round eschtion is sealed [caulked] to the tub surround but the spout is not sealed to the eschtion

 

[Underdog]

Problem solved. Turned out to be the cold water faucet. When the shower was turned on the back pressure was enough to cause the faucet valve to leak down between the surround panel and the old plaster wall. The water would collect on the lip of the cast iron tub and pool there before dripping off the edge and into the basement.