K Heres muh bit.
There all very general questions so its hard to know where to begin. SO ill just ramble on for a bit.
Turbine trims are figure that describes the pitch of the fins, high values like 90 are considered low flow high spool. Lower values like 80 are considered high flow low spool. With regards to question as to how to match turbine a/r and trim to power figures. There is formulas for working it out, but majority of the time the turbine side is fairly evenly matched with the expected max flow of the compressor side in most pre-built turbos. If you go and mix and match wheels then youd have to sit down and do the math's but why deviate from something that turbo engineers have obviously tried and tested. One thing that ive noticed is that turbo theory doesnt always become fact in real world scenarios, a turbo i had built should have flowed well according to theory, but when put on the car was lucky to push a rb25det to 190rwkw. Simply chaning the turbine housing got me an extra 60rwkw.
Wastegate sizing, im not sure if there is any formula for working out minimum size, but obviously it is a component that doesnt become detrimental* when it gets too big. The bigger the better IMO.
Head / manifold design, this is where i dont have much knowledge but I do think that turbo engines are much more forgiving of slight design / construction mistakes than a NA car. In a NA car any stuff ups will make the car work alot harder to get the air down into the cylinder using vacuum, where as turbo crams it down whichever way it can. I think that even distribution is important (std plenum), i think that aftermarket front entry plenums cant be brilliant for equal distribution at higher rpm / flows unless of course they are using internal baffles / channels.
Cams i dont know enough about to comment.
As for the whole interelation. Well there definitely is but its still always a compromise, every part has a specific design goal whether it be responsiveness, outright power, torque... you cant have it all.