Hosafos
If you forgot to say Yaaleh Viyavo during Minchah on Monday and only remembered at night, daven Maariv (including Yaaleh Viyavo) twice, once for Maariv and the second time to make up for Minchah.
If you forgot to say Yaaleh Viyavo during Minchah on Tuesday and only remembered at night, daven Maariv twice, once for Maariv and the second time as a voluntary tefillah. However, since it is no longer Rosh Chodesh, do not say Yaaleh Viyavo during either Shemoneh Esrei.
All the above also applies if you are unsure whether you have said Yaaleh Viyavo.
Hallel
If you are saying Hallel without a minyan, say the berachos at the beginning and end. When saying the berachah at the end of Hallel, the word al is omitted.
Many are accustomed to saying these berachos even when saying Hallel with a minyan. If doing so, say the berachos before or together with the chazzan, and do not answer amen on the chazzan’s berachah.
Repeat the possuk of Hodu after each of the three pessukim of Yomar na (even when saying Hallel without a minyan).
