May one visit the Ohel at night?
While some avoid visiting a cemetery at night, the widespread custom is to visit graves even at night. This applies especially to the gravesite of Tzadikim.
Explanation: There is no source for prohibiting visiting a cemetery at nighttime. Nonetheless, some avoid doing so and perhaps this is due to that the side of Gevura and evil is stronger at night and hence we avoid entering the area. Alternatively, the reason is because one does not recite Tehillim at night until midnight. Nonetheless, regarding the gravesite of Tzadikim it is widely accepted to visit even at night.
Sources: See Divrei Torah 3:44; Nitei Gavriel Aveilus 2:82-10
Can you say Tehillim by the Ohel after nightfall?
This should follow the same general restriction against saying Tehillim at night.
Can One say Maaneh Lashon at night, even though it contains Tehillim?
Seemingly, yes [no different than other night prayers that have Tehillim]. Vetzaruch Iyun.