1) Depending on your long-term goals, find out what the restrictions are on renting out both units. I am pretty sure in Whittier for an ADU (not a duplex) the owner must live in one unit, this is common in many cities for ADUs.
Also, be sure to go to the city during your physical contingency period and make sure you are are gathering data from the right source. I just closed with a client on a property that has a Glendora postal code but officially assigned to the city of San Dimas. Glendora's building codes would have allowed an ADU on their lot, San Dimas does not. This could have been a horrible mistake.
2) I agree with @Jon L., it will make your offer a bit less attractive but with a shifting market, you probably be okay.
3) Any qualified contractor should be fine. Have your contractor walk through with you during your physical contingency period, if you could get them there the day of your inspection, even better. This way you aren't asking them to go out to everything you may make an offer on. If you are already in escrow, the contractor will know you are serious. The price seems reasonable, my client just contracted a GC to convert only a garage for about $80k.
4) There is a difference between a duplex and an ADU on an SFR lot, sometimes differences could be how they are metered, what the setbacks are, and how they could be rented out. Often times the ADU could only be a percentage of the primary residence and no more than a certain square footage. If there is Rent Control or a Rent Stabilization Ordinance, it may be triggered if you build a second unit. Just be sure to check with the city.
Hope that helps! Feel free to reach out.