What are the design limits of a normal tub overflow drains (and their gaskets)

Options include looking for discoloration on surrounding areas and even putting a paper towel or Kleenex in suspect areas in order to detect a small leak that may not have been sufficient to create noticeable discoloration. Depending on the volume, you may be able to hear dripping in order to help trace the leak. And you may wish to reconsider whether the initial diagnosis was correct or whether the repair was sufficient.

Here are three causes that weren't obvious to us that you may wish to consider:

(1) Around the tub spigot: We had a situation in which we had a leak from a bathroom on the ceiling of the dining room beneath it. We re-caulked the tub and shower surround, did a small test, didn't see any water leaking, and thought we'd taken care of the problem. The next time someone showered the leak reappeared. We eventually figured out that the problem was around the spigot: Water from the shower was hitting the spigot, and because there was not a good seal around the spigot, water was running behind the spigot, down to the floor, and eventually into the dining room below.

(2) The actual copper supply line connections--if you're fortunate enough to have access to them without tearing out the wall: In another house, one with only one floor, we thought we had a leak in our shower pan, but during a remodeling discovered that the hot supply line had not been soldered correctly and water had been squirting out between the wall and eventually out through the brick in the adjoining exterior wall.

(3) Diversion due to insulation: We had another leak that we thought was due to a problem with the flashing between the roof and adjoining siding. After tearing up the roof and the ceiling of the room being damaged, we discovered that the problem a poorly-sealed nail in the roof itself. However, this location was several feet away from where water damage showed up. In this case, the insulation directly below the small leak was sufficient to keep the water from running straight down. Instead it wound around and into another room.

First, make sure that the clog is accessible via the overflow. Try this test... While the tub is draining slowly, pour water down the overflow through a funnel. If that drains fast, then the clog is in the horizontal arm and no clearing through the overflow will help you.

What works for me is to use a wet cloth to seal off the overflow (replace it first to make as small an opening as possible). With the tub 1/2 full of water and holding the wet cloth over the overflow vent, a plunger should then be able to clear the clog because it will force water down the drain (horizontal or vertical) rather than pushing water up the overflow pipe (or pulling air down it). Should that fail, use draino or physically pry off the grate for an auger.

The plunger should pull some amount of gunk back up into the tub while you're using it, even through the grate. If that doesn't happen and the drainage does not improve, then you may have a clogged vent. You need air behind water for it to drain quickly otherwise it's like water draining from an upside-down bottle. Cleaning the vent means going up on the roof and clearing the clog from there -- a hose at full blast will usually do this.

As for fully removing the overflow and parts... I don't know.