Can one wear a silk dhoti and perform tarpanam? Should tarpanam be done only while sitting? Or can it be done by standing due to physical incapability for sitting continuously on the floor for a longer time?

While some perform wearing silk, our elders do not have this in practice. Avoid wearing silk, wear washed and clean clothes while performing tarpanam.
Tarpanam should be done being seated; if not use a plank or chair but make sure you do them carefully such that the water added does not come in contact with your legs.