«مُبْصِرٌ» and «وَاعٍ» identify anyone capable of seeing or hearing. Their indefiniteness fits the broad depiction: perception of the praised person's excellences is not limited to one witness.
The construction «أَنْ يَرَى ... وَيَسْمَعَ ...» forms an infinitival clause elaborating on the rivals' grief and the enemies' resentment. Its rhetorical point remains intelligible without a spoken object for each verb.
The central praise is the person's manifest fame: ordinary perception and reports bring his qualities before the world.