If one makes an early Shabbos during the nine days of Aveilus before Tish'a B'Av, does that mean s/he can eat meat earlier? Or does one still have to wait until it is actually nightfall?
On Shabbos there is no restriction against eating meat or drinking wine even if one began Shabbos early - any time after plag ha-Minchah.
An oral ruling of Rabbi Moshe Feinstein (recorded in Moadei Yeshurun pg 131 paragraph 8) states:
One who extends the observance of Shabbos by accepting it earlier on Friday or keeping it longer Saturday night, may eat meat and drink wine or grape juice during the extended period.
The footnote there (#62) references the oral ruling as well as one of Rabbi Feinstein's student's responsa, Rivevos Ephraim 1:367:2, who doesn't cite Rabbi Feinstein (as there is no prior written record of this ruling of his) but comes to the same conclusion only in the instance of extending Shabbos earlier into Friday.
(h/t Rabbi Doniel Neustadt for pointing me to Moadei Yeshurun)