If the mosque is not crowded, should the worshiper stand in line with the congregants between the pillars, or start a new row away from them, and does the ruling regarding prayer between the pillars change given the spacing out of worshipers due to the COVID-19 pandemic?

Congregants are prohibited from praying between pillars except out of necessity. The reason for this prohibition is that the fundamental principle for rows is that they should be connected and tightly packed, and the presence of a pillar breaks this connection. This legal cause (for the prohibition) has been removed with the social distancing among worshippers due to the pandemic, so there is no longer any reprehensibility in praying between pillars in this situation.