I have a work visa (PhD program in Switzerland) starting from December 20 2015-March 19 2015. It is my understanding that during this time, I must obtain my residence permit which allows me to remain/leave/return to Switzerland after my visa expires. I would very much like to purchase a flight on December 15th, 5 days before my visa is valid. As an American, I can travel to Switzerland as a tourist for 3 months without a visa. Would it be acceptable to go there on the 15th as a "tourist" for 5 days until my visa is valid WITHOUT A RETURN FLIGHT? The Swiss Consulate of SF informed me that this should be acceptable, but they cannot guarantee this.
In short, you should be fine.
The type D visa is not a "stay longer than 30 days" visa. It's more like a "stay longer than 90 days" visa. More properly, it is a "national" visa, meaning that it is primarily governed by national law rather than the Schengen agreements. National visas do have a role in the Schenge system, however: They are equivalent to residence permits for the purpose of duration-of-stay calculations under the Schengen Borders Code.
(If my calculation is correct, your visa is valid for 91 days; I wonder whether that's actually how they calculated the end date, or if they were going by the day of the month so the visa would have been valid for 90 days on a non-leap year.)
In any event, presence under a national visa is explicitly excluded from the calculation of duration of stay under the 90/180 rule. The actual wording in the Schengen Borders Code is
1a. For the purposes of implementing paragraph 1, the date of entry shall be considered as the first day of stay on the territory of the Member States and the date of exit shall be considered as the last day of stay on the territory of the Member States. Periods of stay authorised under a residence permit or a long-stay visa shall not be taken into account in the calculation of the duration of stay on the territory of the Member States.
This appears to indicate that you can arrive on the 15th as a visa-exempt traveler, whether in Switzerland or elsewhere in the Schengen area, remaining 5 days under those terms; your "period of stay authorized under a ... long-stay visa" will begin when the visa's period of validity begins on the 20th. At that point, the 90/180 rule no longer applies.
(This of course assumes that you have stayed no more than 85 days in the Schengen area in the 179-day period from 19 June 2015 through 14 December 2015.)
As Relaxed notes in a comment, there's no strict requirement to have a ticket for onward travel. The requirment is that you be able to demonstrate that you will not stay longer than allowed. Your long-stay visa is evidence of that.