First off i do not think the new enemy is the forrunners; it does not say forunners are the new enemy now does it, it say an adversary which might turn out to not be true as you go through the story and chief finds out they are friends (also i don't think a forunner in the flesh will show up in halo 4, mabye halo 6) or something so like that.
reclaimer is not a title for jumans, its a human title, such as commander or gereral. there was a human back in the forunner era 100,000 years ago that helped out the forunners and was called reclaimer by a monitor. the same thing goes with MC. it even says in halo: the flood book that 343 is looking for the reclaimer to help him get the index and that to recieve the title, you need to get to it without dieing.
the reason humans can use forunner technolegy is because of chakas (and possibly the librarian)because of his involvment with it and putting human dna into the controls.
precursors are all dead except for the timeless one. but could be the new enemy. i wont disclose my thought on who the new enemy is, just who it isnt.