2. A FPGA + Cypress FX3 solution can be used. FPGA should convert the MIPI signal to parallel output. FX3 is can take this parallel data as input and send to the USB host (Refer AN75779). You may use the low cost Lattice MACH XO3L FPGA + FX3 to implement this.
3. Using an ISP with two sensor inputs (this may be an expensive solution). Cypress doesn't have any reference design for the same.