1) Going through your points.
A square wave is made from multiple sinusoidal waves. Often in 1, 3, 5, etc. times the base frequency. Now as you mention, the rise time is 2.5ns, so it is not really a square, but a trapezium. It still requires multiple frequencies to make. There are many video's on EMC and signal integrity that shows details. Take for instance this one: https://www.youtube.com/watch?v=qVREULDBtjk
More video's are from Eric Bogatin.
Trace impedance is depending on the dielectric, trace width and distance from planes (microstrip / stripline). So the width may not always be the same to get a 50 ohm impedance. Length matching may not be required much as it mainly depends on the timeframe. From memory, a 100MHz signal can have 50mm length difference without much problems. It was in a video from Rick Hartley,
90 degrees bends are only starting to get problematic above 20Gb/s
400mils = 10mm, signal traveling at about 150mm/ns. So if you need the signals to arrive within 1ns
, you have 150mm length matching tolerance. NOT 10mm! Do note that you already mentioned that the rise time is 2.5ns, so 1ns is less then half of the rise time.
It takes to for the signal to travel from the MCU to SDRAM. As mentioned, the signal travel about 150mm/ns (or 0.15mm/ps). With difference in length, the signals arrive at a different time. The time is more important than the length. The maximum time difference (AC timing) will be in datasheets. Often a trick to use is to make the clock line the longest.
Temperature may be a factor, but as mentioned, I do expect you to have plenty of room for tolerance.
2) See my remarks above on length tolerance.
3) TFT displays are terrible in details. And TERRIBLE in EMC. I once asked Istvan Novak about the Z0 of FPC, but he could not answer. From what I could find it is closer to 70 ohm. Make a good EMC Gnd connection between your board and the LCD display! You may need to a add series termination too.
4) Most fabricators have a standard 4 layer stackup, but they are terribly bad for SI and EMC. See video's of Rick Hartley where he shows better stack-ups (including 4 layer).
5) Yep, that is a good question. I would say it depends on what you want to do. Do you want Altium to calculate the impedance or do you let the fabricator do it for you?
With the first you would need to put in the final size. The problem with that is that if you are not careful, the fabricator may think you want a base copper of 0.035mm and then add the plating (getting to about 0.06mm).
6) Do you need a power plane? Again, see the video's of Rick Hartley about stack-up.
Better a long question that is clear, then a short one that misses a lot of required details...