User blog comment:Hawkinz340/Nineteen Kings and Alabasta Poneglyphs/@comment-13876472-20130925190232/@comment-5962458-20130927130108

First one is easy. The Nefertaris were part of the original alliance but, when they saw the others' plans for world domination, they wanted out. Meanwhile, the survivors of the Kingdom started planting poneglyphs and one of them happened to be in Alabasta. The Nefertari's recognising what major duchebags they had been, so they desided to protect it, in order to redeem themselves.

Now about the bounty. That's a question alright. My guess is, that DD was bored of the holy life, and wanted an adventure so he simply left. That also explains the rag clothes in the kid picture. When he became notorious, he got a bounty. When the marines and WG realized his origins they made him a Sichibukai and forgot about his deeds.