1: At the time of his death, Arad was held by a Lebanon-based unit of the IRGC. It's very possible that Hezbollah knows where he was buried. It's extremely unlikely that the Beirut gov't does. Therefore, who exactly is "negotiating".
2: "in order for the Lebanese army to prove its ability to disarm Hezbollah". .... The Beirut gov't can't do that. The Taif Agreement of 1989 and the UNSC Resolution 1701 of 2006 called for it, and Hezbollah refused. Donald's delegates to those "negotiations" are probably like him - too illiterate to know about any of that.