Please help!!! Is this toxic mold behind a tiled shower? How to fix it?

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Hello, thank you in advance for any suggestions.

We recently purchased a used home (the home was built new about 19 years ago), we noticed a 3 foot diameter painted-over section on the kitchen ceiling directly below the master bathroom shower. After the first use of the master bath shower, a half moon shaped brown spot formed adjacent to the painted over area (on the side of a recessed high hat light fixture). So we knew there must be an active leak in the master bath shower. The shower is a stand-up tiled shower with plastic base pan. It has a Kohler Coralais shower handle which is about 19 years old. We removed the shower handle trim, then manually turned the shaft to put the shower on, during which time we noticed water dripping from the Kohler mixer cap (this part: . The water was dripping about a drop every second right into the cut edge of the sheetrock below. So we turned off the main water supply of the house and removed the Kohler mixer cap and Kolher Cartridge (new parts on order). In curiosity of what sort of water damage this may have caused (and how long it has been leaking prior to us buying the house), we stuck a mobile phone into the shower handle hole, turned on it's camera flash and took multiple pictures of the inside of the sheetrock (which the shower tile is mounted to). Those images are as follows:

It appears there is black colored mold on the inside surface of the sheetrock which tile mounts to. Naturally we are very concerned, is the mold toxic? has it spread to other areas, such as the subfloor and kitchen ceiling below? Can we safely live in the house with mold behind the bathroom tile shower?
So we called a local mold remediation company and they scared the crap out of us telling us that area of the house should be quarantined, we need mold lab test samples, air quality samples, then need to have a professional remediation company come to remove the tile shower and sheetrock behind, respirators, full body suits and googles have to be worn when working around that area. The other side of the wall is the master bedroom which is nicely finished, painted and brand new carpet installed so we really do not want to have to cut open that wall in the bedroom (let alone cutting open that wall may spread toxic mold into the bedroom and the rest of the house). Furthermore, now we are afraid to use the master bathroom at all, reading online makes it sound like this black mold is a serious problem that will spread throughout the house.

So we are looking for some honest input after seeing all those photos linked to above (it was the best we could achieve with the cell phone camera working in that little space around the shower handle mixing valve assembly). Are these problems common? should we be overly concerned? do we need to start ripping out the tile shower and redoing the bathroom? or could we simply spray some mold killer into that area, install the new shower mixing valve parts to stop the leak, put the shower handle trim back on and forget about it?

Here is the mold killer solution and sprayer we can use, any thoughts if this will work? (obviously we can not reach our hands down into the area to scrub anything without ripping apart the wall and tile, just not enough access space around the shower mixing valve).
Mold control chemical -
Pump sprayer -

In the pictures there is also white stuff on the inside of the shower sheetrock, what is that white stuff? We figure the black areas are mold, but the large white areas not sure what that is.

Thank you all so very much for any input or suggestions!
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