If one has offended a bhagavata (bhagavata apacAram), will the resultant sin be removed by atoning through prayschitta prapatti? Moreover, if the bhagavata who was offended has departed for the Eternal Abode (attained Acharya Tiruvadi), how to remove the sin due to an offense committed against that bhagavata?

Sins committed against a bhagavata are the most grievous of the sins; it will make Emperuman’s Divine heart furious. Hence repenting for offending a bhagavata itself is a good start and at once it is sagacious to seek forgiveness from the concerned bhagavata. If that bhagavata has attained Acharyan Tiruvadi, mentally repenting for the offense and must seek forgiveness from Emperuman.
If one’s Acharya assents, he will make one perform prayaschitta prapatti. One can supplicate to one’s Acharyan stating the intention to perform a prayaschitta prapatti sharing the details of the offense committed. If the Acharyan agrees, one can perform the prayaschitta prapatti.