Pretty sure I tested Magic Map and it worked properly with Enemy Sense, but it is always possible I missed something.
Magic Map itself should trigger "fully explored" effects the moment you use it. If that doesn't happen, please let me know. I did test it now and it seemed to work properly.
The character in question appears on floors 18 through 20, if he appears at all (even with Enemy Sense, it isn't assured!)l, and Jayle is not able to have the character as a minion. No other summoner is high enough in the Tower to have theoretically encountered the character, so none of the others will have him either. (Also, as a rule, enemy summoners never "expend" uniques for things like Soul Armor or modifying demons. One of the few times I'm slightly nicer to the player than I am the AI: it's no fun fighting uniques in forms you won't be able to lay claim to as "goodies"
)