SOLIDWORKS Design & Help

I´m currently making a car shell design for a sport car on my next college project, and I´ve been getting a couple of problems.

I can´t make the project curves have the design that I want and once I try to trim the parts that I don´t like to fix them, it either won´t let me or solidworks will crash. I also don´t know how to do those "wavy" parts of the car since I last time I tried the "scoop" got mixed up with the lower parts. I´ve been looking at this tutorial so far, and it´s been helping me a lot, but my car is way more ergonomic and maybe that´s why I can´t do the same things(??)

https://www.youtube.com/watch?v=Cb-MBo75Oc8&t=255s

And the last thing is my car has a very "unique" (for the lack of a better word) design up top (last pic) and I don´t really know wich option is the best to make it, should I make splines all around and fill averything with boundary surface, loft etc. or should I just make each part individually and mirror it after? should I go piece by piece, or is it better to make it whole?