(LEAD) Speculation mounts on Cuban visit by North Korea's No. 2 man
(ATTN: CHANGES headline, dateline; ADDS comments of Seoul experts from 9th para)
SEOUL/MEXICO CITY, Aug. 17 (Yonhap) -- Choe Ryong-hae, known as North Korea's second most powerful man, is visiting Cuba to discuss ways to improve the countries' bilater