The tile installers didn't shut off the shower valve when they left after installing tile work in our shower. We turned on the main water supply and 15 minutes later, we went to the bathroom to check the tile work. The bathroom was soaked and water overflowed in our master bedroom. Do we need to re-install the tiles? Also, we're wondering if it is necessary to remove the backer board to check for water seeping behind the walls. We've placed a dehumidifier the entire night to remove all the moisture and to keep the bathroom dry.
There are two problems I can think of. The first is exposing the thinset to excess moisture, this will impact the chemical reaction that's part of the curing process, which may lead to a weaker bond and loose tiles. It's easy enough to wait at least 24 hours and then check the tiles to see if they are firmly in place. If a bunch are easily dislodged, then I'd remove everything to inspect for damage behind the walls. You'll want a new backer board for a flat tile install anyway.
The bigger problem is if water got behind the tiles into the wall. A proper install will have a waterproof barrier behind the tiles since grout, thin set, and often the tiles themselves are not waterproof. If this barrier (e.g. Ditra) isn't the case in your install, the materials behind the tile and possible paths water could take are important. If water is getting into drywall, even green board, wood (studs, plywood, or OSB), or on metal (non-galvanized nails/screws, electrical lines, communications lines) you will see damage. The damage will likely take time to detect, e.g. rusting, corrosion, and mold won't be noticeable immediately.
On the flip side, it's also possible that any damage that has resulted will not be noticed until after you sell the home or perform another renovation. Because of this, it's difficult for anyone to give a definitive answer. You may be exposing yourself to significant health hazards from mold, or you may not have any issues at all.
I don't think this is an issue. You aren't blending the water into the mix. You are basically introducing the water to the outer skin of the thinset. Yes some may get into the "inners" but I believe this would be a minute amount and I personally would be cautious but not worry too much. I might delay grouting for a few days and thoroughly test the tiles after 2-3 days but really this should be no issue unless the tile job was flawed to begin with.
Also common sense says they started at the bottom and worked their way up. So the tiles (that got the wettest) would have been setting a while. Again I think that if there was a tile problem it was probably more indicative of the job they did before the water turned on.