How does SC-FDE modifies $E_b$ (in AWGN function)
Here $E_b=N$ means $N$ pulses (i.e. PHY waveforms) of unit energy represent one information bit.
I suppose it is due to the S/P (serial to parallel) block that translates each bit of information to a vector of size $N$. So it is implicitly assumed that the energy of each component of the vector is one.

Regarding your second question, you should consider the number of information bits. If you have one bit of information to generate the two vectors of size $N/2$, then again $E_b=\frac{N/2+N/2}{1}=N$.