How can I calculate CM and DM gains for this model?
Think about the voltages at nodes \$b\$ and \$d\$, and the voltage divider formed by \$R_s\$ and \$R_{in+}\$. For simplicity, it's assumed that the amplifier gain is 1. Noticed that the gain does not affect the CMRR calculation because it shows up in the numerator and denominator.

Differential mode:

no noise => \$V_e=V_d=0\$; \$V_{in}=V_a\$

\$V_{out}=V_b-V_d=V_{in}\frac{10MΩ}{10MΩ+10kΩ}-0\$

\$\frac{V_{out}}{V_{in}}=\frac{10MΩ}{10MΩ+10kΩ}=0.999\$

Common mode:

no signal => \$V_a=V_e=V_d=V_{in}\$

\$V_{out}=V_b-V_d=V_{in}\frac{10MΩ}{10MΩ+10kΩ}-V_{in}=V_{in}(\frac{10MΩ}{10MΩ+10kΩ}-1)=-V_{in}\frac{10kΩ}{10MΩ+10kΩ}=-0.000999\$

For CMRR purposes, we don't really care about the sign of the gains, so we get:

\${CMRR}=20log(0.999/0.000999)=60dB\$