Adiyen usually does Mahalaya paksha tharpanam on Shashti, or Madhyashtami, or on Mahabharani. This year, Adiyen has to participate in Pitru sthanam for my maternal uncle vashabdikam. Can Adiyen perform Mahalaya tharpanam on Madhyashtami? If I sit for the pitru sthanam, how many times should Adiyen recite Gayatri japam?

According to the scriptures, one should not take part in Mahalayam rituals involving rice that are performed by others, unless one has performed Mahalayam themselves.
Maternal uncle’s varshabdikam is not in question. This year, for followers of the Sri Munitraya tradition, Srijayanthi is on Madhyashtami. On that day, they must not perform Mahalayam. Followers of Sri Sannidhi can do Mahalayam on Madhyashtami.
If you participate in varshabdikam, 300 Gayatri japam must be chanted as atonement and must not be at another place for 3 days.