I know that changing a name on a reservation even a sinlge letter forces Shares to reacquire new APIS data. Not sure how UA.com would handle this, or if it would created more problems (etkt no longer matching name) than it would solve.

I appologize that I am of no help here, as I deal with reservations of people already in the US, so the 1st night stay address is moot from my perspective. AFAIK, the phone number of 1st night stay is not part of US APIS info, rather address is, so it isn't a part of the requirement, and not something you need to worry about.

The required fields are: Full name (last name, first name, middle name if applicable)
Gender
Date of birth
Nationality
Country of residence
Travel document type (normally passport)
Travel document number (expiry date and country of issue for passport)
[For US travellers] Address of the first night spent in the US (not required for US nationals, legal permanent residents, or alien residents of the US entering the US)

So my advice, don't sweat it, it isn't part of APIS data. A res agent can add a phone field at will, just becuase it doesn't show up as APIS should have no impact on you in the least.