Bad Reflection from SMA Board Connector at 4-8 GHz
In order:
a)  Yes.
b)  No.  A classic would be not realizing how bad FR4 is at 8 GHz, but you've covered that.
c)  Yes.  Angles of any kind are bad at these frequencies, and while 90's are not the worst, they are a contender.
Note that the microstrip structure is a compromise, and always will perform more worse (?) that a simulation, compared to the Sim/IRW difference of a full stripline design.