Does sitting in the mosque between Maghrib and Isha prayers count among the deeds that will put one under the shade of Allah on the Day of Judgment?

You should praise Allah Almighty for His enabling you to do good and seek more of His blessings and steadfastness in obeying Him, because steadfastness is one of the greatest means of entering Paradise, as stated in the verse: "Indeed, those who say, 'Our Lord is Allah,' and then remain steadfast - there will be no fear concerning them, nor will they grieve."

Regarding your question about being among those whom Allah will shade in His shade on the Day when there is no shade but His shade, seven types of people are mentioned in the noble Hadith (definition). However, it is not possible to definitively state that you are among them because it pertains to the unseen, and deeds are judged by their conclusions. The Prophet, peace and blessings be upon him, disapproved when Umm Al-Ala' stated with certainty that Allah had honored Uthman ibn Maz'un, despite his hope for good for him.

Continue to be steadfast, hope for Allah's mercy, fear His punishment, and ask Allah to make you among those whom He shades in His shade.