To answer your questions in the following bulleted order:
- Yes, the MODE/CON connection is correct.
- Assuming that you want PWM mode, having more than 1.2V on the voltage divider would suffice by dividing your input power in this case.
- If you leave the voltage open, you will be in PFM mode, which is better suited for light loads which it seems is not the case for your 3.9V @ 3.5A output application.
Unfortunately, the only reference that we have in the datasheet has been posted in Figure 8 above.
But I would say that using the MP2145 would be best to give your output current some headroom. Also, it is worth mentioning that your efficiency would certainly be higher when operating in PWM mode.
Best,
Krishan