For a definitive record is it possible to determine if a namecoin block was generated via merged mining? Is so which was the first one.
Which is the first bitcoin block to have namecoin information in coinbase transaction?
Yes, block 19200 was mined using MM, by slush's pool. AFAIK first Bitcoin block with merged-mining information was 00000000000002c727047296ee20b628599031c6ea5c09292513fddbb11d34df (block 148557), also mined by slush's pool.
NMC block 19200 was the first block to have merged mining hashing on it, Though I don't know if a merged mining pool found this block. In merged mining, your submitted shares are passed to bitcoin to check if it solves a block. Then it is passed to namecoin. Each are independent of one another. If you find a NMC block, it's just that! No BTC are generated!