Does praying in a place that combines sunlight and shade affect the validity of the prayer? What is the ruling on previous prayers performed in such a place? And does the ruling of disliking sitting between sunlight and shade apply to sitting between light (from a lamp, for example) and shade?

It has been established that there is a prohibition against sitting in a place that is between the shade and the sun, because it is a seat for Satan, in addition to the health implications that result from that. This prohibition is understood as one of dislike (Karahah) rather than outright prohibition (Tahrim), so prayer performed in such a place is valid.