Legally? No. Copyright is copyright, regardless of distribution. "Fair Use" doctrine is something people like to throw around because they're not making a profit, but the two are unrelated and it doesn't apply here. That "for entertainment only" thing you see on infringing Youtube videos? Complete BS, it's legally meaningless. Microsoft have complete and final say over who does what with the Halo brand, characters, story and asset designs. Assigning credit is also legally BS, the only time it's relevant is part of a licensing term on some kind of media contract. Sometimes it reduces the licensing fee, sometimes it's just part of the standard package. It's usually a sign of respect though, knowing and passing along who actually owns the IP.
That said, it's a similar "grey" area to propmaking- the assets and branding aren't legally ours to do anything with, but for the most part, because it's so small-scale, it gets ignored. If you ask them nicely first, their legal team must advise them to say no or risk having their trademarks diluted and future copyright cases (against mass Chinese importers, not random Joe) damaged. They cannot be held by a court to have had actionable knowledge- not that it would probably matter for a multibillion dollar US company for an FTC injunction with the lawyers they have, but they have to play it safe or risk shareholder revolt.
So, if your game proves to be not particularly popular outside of the community and you keep it reasonably small and simple (not quite Mister Chief Warthog Explosion simple, but reasonable)- and, yes, assign credit all over the place- then you'll probably be left alone.
But if they issue a takedown, it's your responsibility to comply. Since your work is your copyright, you could always just pull a 50 Shades and magically transform from fanfic to franchise. Dump the characters and assets that are theirs, remodel where you have to, retexture where you don't, redo some story and audio... It would be fundamentally identical, but different enough to comply and you'd still have your work out there.
Isn't Source 2 out soon? Pretty sure Unreal Engine, Havok and Unity are all either free to use or free to get started with too. Could be good, may take a while without 5-10 other people helping out though!
