Is online Tharpanam (performed by proxy) spiritually valid?

Yes. The Dharmashastra tradition has always permitted proxy performance by an authorised Brahmin when the Karta is genuinely unable to be present — this is called Pratinidhi (representative) performance. The essential conditions are: the Karta must state the Sankalpa (even via video call), the Brahmin performing must be qualified and sincere, and the correct materials and mantras must be used. All three conditions are met in our service.