Haj Amin is not the only one at blame, and I didn't blame the Palestinians for him. You know he was intriguing and whipping up trouble long before becoming Mufti. As a delegate to the Syrian Congress he helped pushed through a resolution stating that Palestine was the southern and indivisible part of Syria. He also demanded a complete rejection of Zionism and whipped up riots against the Jews in Jerusalem, you can't say that he forced them to riot can you. The violent outbursts of 1929 were even worse.
Herbert Samuel was acting according to official policy, which was to mollify the Palestinians as much as possible. He thought that by making him Mufti, Haj Amin would mellow. Until him, Jersualem was a backwater among Muslim holy sites, despite being the site of Muhamad's Ascension according to tradition. He used all his power and influence to change this.
Husseini's family was powerful and had much influence, they were among the Palestinian elite for generations the Palestinians were pretty much resigned to them. There wasn't much protest over the apointment.