You're asking questions that aren't going to have one-size-fits-all answers. This likely comes from being anxious about the possibility of owning an X. Such anxiety can make you forget it is just another vehicle. The simplest options are to jump in feet first and plan to make it work or to find an X and do a test fit. Taking your current route a step further and asking someone else for dimensions isn't necessarily going to help if you get a different configuration than theirs or if they measure differently than you. That having been said, I'm going to try to answer your questions to help support these points.
Our forward facing car seats were rather large and would NOT fit in the 3rd row, but it seems incredibly unlikely that Tesla would install the LATCH system in the third row if there weren't car seats out there that would fit.
I'm not sure what car seats have to do with luggage or strollers. I have a family of 6, and the youngest just graduated from her car seat. Removing the car seats didn't change the amount of space available for luggage or strollers at all.
There is some space below the floor behind the third row, there is some space behind the third row seats when they are up, there is some space in the frunk, there is some space in the floor below car seats (and unoccupied seats) in the middle row, and there is space in any actually unoccupied seat. Some luggage and some strollers will fit in some of these spaces, some luggage and some strollers will not fit in some of these spaces, there are likely luggage and strollers that will fit in any of these spaces, and there are likely luggage and strollers that will fit in none of these spaces.