If a person has to say (or finish) Slichos after Shachris on Erev Rosh HaShanna, is Viduy and Tachanun (of Slichos) said? Can Slichos be said after Chatzos HaYom on Erev R.H.?
Yes. Although the optimal time for Slichos is before dawn, many say Slichos after daybreak. Here too, there is a preference to say the Slichos as early as possible, but it is permitted to say them later, too.
Even when davening alone the full Slichos are recited (Mateh Efraim 581:22), with the exception of the Thirteen Attributes that are not recited biychidus.