TSQL - merge partitions online
I solved this situation using a simple workaround (very similar to DanGuzman's comment).

Merging partitions is in-blink-of-an-eye operation if and only if both partitions are empty. When the partition function is defined as RANGE RIGHT, then during the merge the leftmost partition will eat the other on the right side. Due to this all records have to be moved between partitions filegroups. The workaround is to rebuild or simply drop and recreate the staging table - the table with exactly the same schema as the source partitioned table. The only difference is the filegroup of the nonpartitioned table and it's indices must match filegroup of the partition that is going to be merged. This reduces both of operations - partition switching and partition merge to be metadata operation only. QED.