Do you have an upstairs tenant or the sewer is backing up.
Your kitchen sink drain and your shower drain are connected to a common drain line. There is some sort of blockage in the drain line past the point where the shower drain ties in. You need to call a plumber to run a snake through the drain line from the kitchen to where it ties in to the main drain of your house.
The drains for the shower, toilet and sink all connect to a common line. Assuming the toilet is not backing up, the problem indicates that there may be a partial blockage, enough so that some of the toilet flow is backing up through the shower drain line. You need plumbing repair. The shower leak you mention should only be in the shower, or else you have a broken drain line as well.
The shower drain and waste lines on up to the other fixtures located above need to be clear so that water will flow. Just downhill of the shower drain there is probably a blockage in the waste pipe. The blockage could cause slow draining. If it is completely blocked, the shower pan will not drain back out over time. You will need to run a plumbers snake down the line below the shower drain. If the toilet in the same room as the shower when flushed also shows water coming into the shower drain, it will be best to take up the toilet off the base and run a plumbers snake from that point. Warning for very slow draining lines or lines that do not drain, DO NOT use Drain cleaning chemicals. Good luck and you may wish to call rotorooter.
First determine what you are going to install as the shower unit. If there is to be no shower unit and it will just drain through the concrete floor, then it will be flush with the floor. If you are installing a unit, then you need to look at the unit to determine the distance obove the concrete to place the drain.
You have a partial blockage downstream of where the shower drain & the sink drain join into one pipe. You could try a thin plumber's snake, or add some drain cleaner, some bleach, or vinegar. About a liter of vinegar is the safest and should work well enough. Davis (davisbigelow.blogspot.com)
You put a drain auger down the pipe from the nearest cleanout. You will feel the blockage when you get to it, - then break it through and clear it with the auger.
Generally it is caused by either a blockage or partial blockage in the drain downstream. But I have known backfall on the drains leading away from the toilet to cause it as well, though this is rare.
Shower drain goes into a trap under shower. Continue piping from trap to drain line.
cut the opening on a 45 degree angle and caulk in drain
The flange connects to the shower and just goes through the floor. It shouldn't mount to the floor.
Water won't drain from shower. Either trap or shower drain line is clogged.
Arachnoid villi can be compared to the drain in your shower or sink. Think about a wad of hair covering the drain. The water cannot drain out and if more water is added without removing the blockage, there will be a build-up of water. It is the same with your arachnoid villi. If blockage occurs, the cerebrospinal fluid won't be able to flow freely, thus building up pressure. This is called hydrocephalus or "water on the brain".