CAMBODIA LOSES KEY MEKONG CITY
Neak Luong falls after heavy onslaught by Communist-led insurgent troops; had been under seige for more than 2 mos; Banam, its twin city, also falls; fate of at least 3,000 Govt soldiers, 40,000 civilian residents and 20,000 refugees in city at time of collapse is unknown; strategic consequences of insurgent victory assessed; map (M)